I just purchased a used Great Planes Big Stik 40 with an OS 0.46LA and the previous owner ran 10% nitro..I use 15% nitro in my other plane, will it be ok to use my 15%? Or will changing the fuel cause problems?

No problem at all so long as you retune the carb to suit whatever fuel you want to run.

thanks alot, i have had a bad experience with a nitro car i bot changed fuel, it ran 3 tanks and blew up.

You'll be fine with the the 15%. As was already said, you'll need to reset your needle. But other than that you shouldn't have any problems

Ken