I can't imagine that a 7 wt. line would damage your 6 wt. rod. I regularly upline my 8 wt. to help with wind. In fact, I stopped buying 8 wt. lines because it is pretty much always windy on the ocean.
Uplining is a common practice, but its effects are highly dependent on the specific rod and the person using the rod. The only way to find out if it works for you is to either borrow a 7 wt. line from someone or buy one. It won't damage your rod.