This came up in a recent game. After I claimed the battlefield, a character with personal shield was about to take 2 range damage. This met the condition for the "before" ability so I tapped the upgrade to take a shield. My opponent said I could not do that because my turn was over since I claimed the battlefield. My position is I agree I cannot take anymore ACTIONS, but condition on the card is a "before" ABILITY, not an action, who's trigger condition was met so it is a legal play. Ruling?

You can do EVERYTHING that is not actions. You can even play cards if you have the ability that lets you do it without taking action.

Yes, you definitively can. Your turn ("round" would be correct) is not over, you just have to pass on all your actions.

Deleted.