[Flyby]

Read my comments higher up , Immo...

And how are you going to trigger that armor upgrade?
Manually, through a PLAYER_UPGRADE ?
not very convenient and a lot of micro-management.
Remember that you also need to switch back to normal, once the unit is no longer garrisoned.

[AntiSocialKindaGuy]

Can you trigger an upgrade when a unit is garrisoned and ungarrisoned?

[Flyby]

Not to my knwoledge...
You'd need a line like "TriggeredBy=", but none of the "contain" modules has it.

You can always try to put it there, as it is one of those super-tag lines that occur in many modules. Yet I'm pretty sure it will crash the game.