Actually, you can have both. You could have that Soviet infantry can be upgraded to "guard_infantry" but that you can also buy "shock_infantry". They may be actually the same except that the former is an upgrade and the latter is purchased directly. Can you ask Claude what was the most elite between Guard and Shock armies in the Red Army during World War 2?

This is what Google AI says when asked that question:

In the Red Army during World War II, the Guards Armies were considered more inherently elite than the Shock Armies.
While both were highly formidable and significantly outperformed regular All-Arms field armies, they represented entirely different organizational philosophies.

The Distinction Between the Formations

Guards Armies earned their status through battle merit.
The "Guards" designation was an honorary title awarded by the Soviet high command (Stavka) to existing units that had demonstrated exceptional courage, skill, and heroism on the battlefield. Over time, these highly decorated divisions and corps were clustered together to create whole Guards Armies. Because their status was a direct reflection of combat proficiency, they were universally viewed as the elite institutional backbone of the Red Army.
Shock Armies were defined by their structural role, not their prestige.
A Shock Army (Udrarnaya Armiya) was designated as such from its creation based purely on its specific tactical purpose. Built to execute the first phase of the Soviet "Deep Battle" doctrine, their purpose was to act as heavy battering rams to breach fortified German defensive lines. They were packed with massive amounts of field artillery, heavy mortar regiments, combat engineers, and assault infantry to create a breakthrough so that more mobile forces could flood through the gaps.

Operational and Structural Realities

• Resource Priority: Both types of armies received equipment priority over standard armies. Shock Armies often received heavy emphasis on firepower (like heavy self-propelled guns and siege artillery) needed for breakthrough operations. However, by the later stages of the war, Guards Armies typically matched or surpassed Shock Armies in terms of qualitative artillery and armored support.
• Personnel Quality: Guards units attracted the best replacements and maintained an institutional culture of high morale and rigorous training. Shock Armies, while heavily reinforced with personnel, suffered devastatingly high casualty rates due to the brutal nature of frontal breakthrough operations, meaning their ranks were frequently replenished with regular conscripts.
• Perks and Incentives: Soldiers serving in Guards units enjoyed distinct institutional privileges, including a unique badge, specific uniform tailoring, and higher pay scales (double pay for enlisted men and 1.5 times pay for officers). Shock Armies did not automatically receive these systemic personal benefits unless individual sub-units within the army had also independently earned Guards status.
Ultimately, a Shock Army was a temporary or specialized tool designed for an incredibly violent tactical job, whereas a Guards Army was a permanently recognized elite force comprised of the Red Army's most proven veterans.

I love how the AI starts by saying

Adolf Hitler did not actually grant elite status or specialized training to every standard infantryman

and then proceeds to explain how he did formally grant elite (grenadier) status to every standard infantryman.

By the way, this could be alternatively (or also) represented by making the Germans unable to buy regular infantry as well as anything lesser, so making them able to buy only "elite" or "grenadiers". This was actually formalized late in 1942. I will let the AI explain better.

If I tell Google AI "Tell me how Hitler gave the elite status of grenadier to every infantryman during World War 2.", the answer is

Adolf Hitler did not actually grant elite status or specialized training to every standard infantryman; rather, he executed a massive, cost-effective propaganda and morale-boosting rebranding campaign.
On October 15, 1942, Hitler ordered all standard German army (Heer) infantry regiments to be renamed Grenadier Regiments (Grenadierregimenter). This sweeping change relied entirely on psychological warfare and historical prestige rather than changing structural reality:

The Power of "Branding" & Historical Illusion

Historically, "grenadiers" were the physically imposing, elite assault troops of the 18th and 19th centuries, famously associated with Frederick the Great’s Prussian Army. By decreeing that every basic foot soldier was now a "grenadier," Hitler sought to link the struggling, heavily fatigued Wehrmacht landsers to a legendary lineage of unstoppable military heroes.

Boosting Crashing Morale

By late 1942, the German army was facing severe strain, immense casualties, and an agonizing winter stalemate on the Eastern Front. The rebranding was designed to give the average, exhausted "straight-leg" foot soldier a psychological lift, making them feel like chosen shock troops without the state having to invest in new equipment or extra rations.

Masking Material Shortages

Simultaneously, Germany's mechanized and motorized infantry regiments were renamed Panzergrenadiers. This created a linguistic illusion of overwhelming armored might. While a Panzergrenadier sounds like a high-tech, heavily armored soldier, the stark reality was that Germany faced severe fuel and vehicle shortages. In fact, only about 11% of these "elite" Panzergrenadier battalions were actually equipped with armored half-tracks; the vast majority relied on standard cargo trucks or simply marched on foot alongside horse-drawn artillery.

Late-War Desperation: The Volksgrenadiers

As the war turned entirely against Germany in 1944, the regime took the naming scheme a step further by creating Volksgrenadier ("People's Grenadier") divisions. Though the name implied a fierce, elite vanguard of the German people, these units were actually understrength, desperate defensive formations. They were rapidly cobbled together from raw teenagers, older conscripts, and transferred personnel from the Navy and Air Force who often received as little as three to six weeks of basic training.
Ultimately, Hitler's sweeping upgrade of the infantry to "grenadiers" was a semantic trick—the battlefield duties, lack of transport, and standard gear of the average infantryman remained exactly the same, but they now carried a title designed to sound formidable to both themselves and their enemies.

@KurtGodel7 Well, "Guard" would be the most typical pick for elite Soviet infantry but would not be a very good one for something which you purchase directly: Guard status is usually earned on the field, so that would really only make sense as an upgrade. What you are searching for is more like the Shock status (ударная армия), which is actually top-down elite from day one. However, that is primarily correlated to having more organic artillery and such: I don't know how much that would (or would not) relate to the infantry itself being elite, though I do remember to have read somewhere that they were paid more than regular infantrymen rank-per-rank (cannot give a reference and may be wrong), and I guess they are also better trained and equipped as a matter of infantry weapons, but I don't actually know (guess you can make a search or ask Claude).