Chances are, champagne yeast won't be able to eat anything the ale yeast didn't already. If it finished too high, then it's more along the lines of the recipe you used. You can try using champagne yeast, but I seriously doubt it will do anything for the batch.
I made an old ale early in my brewing life. I wanted it to finish with a lower FG, and added a packet of EC-1118... Didn't do a damned thing to the FG. Carbonated up quickly, in bottles, but otherwise zero benefit.
Proper recipe formulation, and yeast selection, will do MUCH more for the batch than trying to band-aid it with champagne yeast.