The design of unit upgrades is a bit broken in FE.
I should be able to:
(1) increase the number of units in my squad. (LH reportedly fixed)
(2) upgrade regardless of what the original design called for.
(3) upgrade to something less than the best.
(4) upgrade parts of armor without upgrading everything.
(2) I absolutely hate that I have to remember to select the upgradable option when designing units to have the option to upgrade that gear later. Sometimes I forget. Sometimes, I don’t have the resources. Maybe I didn't have the crystal to build all my units with monk’s robes when I designed them, (opting for leather instead). but now I just defeated some crystal monsters and to upgrade the unit. Chain mail requires a lot of metal, once I researched it I was constantly scaling back my designs of the best to some mixture based on what I could actually afford. This is all fine and dandy when I created them, but then later I can’t upgrade their leathers to chain as I get the resources. (after experimentation upgrades from substandard items work see post below. This complaint only applies when selecting no armor.)
(3) Full upgrades can often be too expensive. Sure I’d rather my unarmored dude rather have chain mail, but when I don’t have the metal I’d like the option to give him leather instead. Or when I research robes after leather I’d rather not be forced to spend my precious crystal on the robes for the upgrade.
(4) Have a squad outfitted in leather and want to upgrade it all the chain- I hope you have a lot of metal. I’d like the option to upgrade piece by piece over time. (usually before a tough fight).