For bittering hops, all you're really interested in is the AA it contributes. Of course, bittering hops will contribute a nominal amount of character to the hop profile, but it won't be substantial. Magnum is generally regarded as a great bittering hop because it is very clean---meaning it contributes plenty of bitterness without messing too much with the hop flavor/aroma profile---leaving you with a clean slate at the end of your boil to add the aroma and flavor hops of your choice. Perle, not so much...but in your case, I would just fix it so the AA's line up, and then go for it. You will, in all likelihood, not notice too much of a difference.