Very simple to add and very important since the game involves some really gigantic maps/builds now:

LEFTSHIFT+MOUSE1 to upgrade all current traps matching current trap (i.e. 10 base traps all upgrade the same with 1 click)

and

LEFTCTRL+MOUSE1 to upgrade all traps of all lower tiers to current upgrade (following current trap upgrade path).

You could also very easily allow the same behavior to affect trap selling (LEFTSHIFT sells all identical upgrade matches, LEFTCTRL sells all of the base tower type regardless of upgrades).

This should be easy to implement and hardcore players really need this feature.