[Mobhack]

There aren't, but there are upgrade path restrictions.

e.g. guns to other arty types, but some of the SP-arty can then morph into a tank type with a second change. Infantry to tank may take some doing - and each change will lose you XP, and the leader ratings will be infantry biased instead of armour.

None of those are documented (it would be far to tortuous to try to explain and 99% of end users would simply be confused).

Basically the best way to find out what can and cannot be done is:
- in a running campaign, save off at the final turn, just before the AI throws the towel in to return to later on (I usually save just before pressing end turn in any case. Helps with the "oops! - I forgot to deal with C Company down South" moments)
- in the upgrade screen, play around to your hearts content with allowed upgrades. This is an experiment, you aren't going to go further. Take notes on a piece of paper/text file etc.
- continue onto the forthcoming battle, and once you get it, don't buy support, and simply exit. Then reload the save you made above
- ditto, if you get a special battle which does not allow fixing just continue onto the next battle's set-up and hit exit, reload the save and continue from there.

Once you have figured out what path changes are needed to get what effect you desire, reload the saved game and this time (read your notes!) you make the requisite changes and proceed with the LC.

However, changing from a totally different formation over to an unrelated one (cavalry to Pzgr say) unless you change the horses/MC to tracked APC and the grunts to appropriately sized teams to load in them - its probably simper to have bought (say) a truck borne grenadier formation and upgrade it to 251s later, or to delete the old one and replace. The latter may be the best route for the Germans - simulating losing experienced guys to form other forces, and you get some new drafts in as replacements.

cheers
Andy