With the recent v13 changes allowing unlimited upgrades, I think it would be interesting, and I think it would make sense, that the Weapon/Shield upgrades that apply when the Unit is produced should stay with the Unit. e.g. If I produce Units with level 10 upgrades and put them up for sale, and a low level player buys them, they get the benefit until those specific troops/ships are destroyed.
It would obviously complicate things a bit in the back-end DB, a Table would be needed to track individual Units and their Upgrade-Levels, Hit-Points s etc., but I really think it would add a lot to the game-play!
The upgrade levels would also need to be listed out when putting troops up for sale in the Black Market. Obviously Units with higher upgrades would demand a higher price in the market.
Spying troop levels would become slightly less valuable. With the unlimited upgrades you need to spy out Inventions already, but if the upgrades are tied to the units then you wouldn't know if the player has purchased higher-level units from another player, which I think improves the game with the additional imperfect information.
The unlimited upgrades already heavily skew the game in favor of higher level players, even more so than before.
Allowing lower level players to purchase high level troops would balance out that equation a little bit, and it will increase Trade activity in the Black Market, which is good for the game.