they just have to read?
http://www.network54.com/Forum/47207/thread/1450506373/last-1450741895/View+Thread
This shows the different weapons that can fire it...as an example
http://www.history.jp/images/D435.1-Pak38.97-de.jpg
A good example is the KWK 40 cartridges that call out StuK 40 also. That means any KWK 40 L43 or L48 or StuK 40 L43 and L48 weapon would use that ammo. In the case of other 'L48' weapons (Hetzer or JgdPzr IV), the ammunition could come with a pamphlet that has the addendum. So much ammo is produced and it takes awhile for it to get to the end user. Weapons might be introduced and the plant would have to adjust the stencils/documentation for these weapons.
I believe that Allied intel would get wind of the ammo types from the documentation that they ship with. That is, they might report that a new APCR round is available for an enemy weapon, but they have not actually captured any yet. I believe this was the case for Pak 40 APCR in some US documents.
In the case of the 'two' Pzgr 40 rounds for the 5,0 cm L60 weapon, the only difference appears to be the powder type. That is, the weight is different but they are the same as far as performance. In the 7,5 cm weapon class, this is common for HE and HEAT and Smoke ammo to have two different types of propellant listed. But it is not used for the PzGr 39 or PzGr 40 ammunition for 7,5 cm weapons like the Pak 40 and KWK 40. That is, there are just one weight listed, an example would be Pak 40, PzGr 39 2.73 kg.
Basically Tanks 'bombed up' the AFV and reviewed the information on the cartridge case and projectile itself as loading. They might stow 'fresher' dated ammo deeper inside and get older rounds in the ready racks. They might have a SOP as far as when they could fire Tungsten Carbide ammunition. Anything 'new' would certainly be investigated and brought up with the technically proficient NCO's etc. when your life depends on the details, you can take a deep interest...