if i drink a + smithing potion will i be able to upgrade an already upgraded piece of gear?

Nope.

Yes you can iv done it.

Huh. Guess I could be wrong

anyone confirm one way or the other? im afraid to test it on my krosis mask

It's important to note if you have legendary gear you can upgrade the stats but it will still be legendary. Do you can have a dedric dagger (legendary) with 35 dm and upgrade it to a dedric dagger (legendary) with 50 dm. So the stats will go up but it will still say legendary only because this is the highist level word. You can however upgrade (fine) to (legendary). But when using potions it's important to look at the stats and no the words because they can be missleading.

You can "re-upgrade" any item as long as the result will be better than before. Or in other words, upgrade whenever you find that you can (because of higher smith level, better smith gear and potion or when you acquire the ancient knowledge perk).

i just care about the stats. I'm just wondering if i upgrade to legendary with say a +40% potion and then later come across a +50% can i upgrade again for the difference?

someone told me that potions don't affect the ability to do this.

Yes you can. Go ahead and upgrade away. Remember though that unlike oblivion the game will not pause effects while in these menus so if it says 30 seconds you have 30 seconds in the menu so make sure you know what your doing and what pices your upgrading and what materials you need.