not sure I understand this, you mean you open a box and the coins are drained from your account and the unit auto-upgraded? That would obviously not work, so there is some misunderstanding here.

Basically, the idea is that there is some value where upgrading is purely positive for your army possibilities (that is some word salad, but I mean like, if you have 8 pawns, upgrading means you only have 3 to use, but if you have 13, you still have 8 to use if you upgrade once).

So in my hypothetical, that surplus would be automatically paid for (incidentally, this auto-upgrade system would save a lot of time lol). In this case, I would then feel free to experiment with any unit I "have", rather than feel incentivized to not upgrade a unit in order to save a few thousand coins. It would be a real 'choice' if it was like upgrading to get a Vampire+++ OR Angel+++ and I had to choose which one I wanted more, but I don't feel like that with getting like, a Temperance+++, where the choice is between "I wanna try it out, at least" vs. "I want to save a couple thousand coins". Even if the coin cost was minimal I'd feel a bit annoyed paying a couple hundred coins for it, it's like fun vs. money-efficient.

Being another market mechanic, Crystals for upgrades may help if it was like, only for upgrades (and maybe regenerating?). Adding choice in upgrading is nice, I think, since there is limited option to choose between getting the copies of Angel or copies of Vampire right now (you basically get equal amount in basics or ultras), but not like this, because I just instantly upgraded to Vampire+++/Angel+++ as those were both things I wanted.