I really like collecting every magic item in the game and I feel like I have to say it but 5e item progression curve is way flatter than previous editions, in 3.5, you could go and stack an item with up to +10 worth of bonuses and it would just cost you 200k... now, a +3 sword is considered amazing and I don't know if I should feel old for complaining about this king of changes since 5e has been around for 10 years already...
anyway, my point is that you should not worry about your items being useless, this game has common, uncommon, rare, very rare and legendary ( white, green, blue, violet, gold for the colors ) rarities and it is roughly decided by the number of bonuses applied on it ( it is not calculated, it is written in the item definition )
now, for the idea of upgrading items, d&d has rules for that and it takes weeks and gold just for adding mundane properties, up to years ( up to 55 years and 500k for a legendary item according to the DMG ) so, I don't think this is in the realm of possibilities for our characters.
finally, if you worry about your adamantine armor being obsolete, just wear a helmet and it already is since the benefit of the minor property "adamantine" is "immunity to critical hits".
too bad they choose those two armors, but I guess giving us an adamantine full plate was a bit much and the half plate isn't even ready yet.
