If you don't want to replace the flasher relay you'll have to install additional resistors which is a more complicated and messier option. A good relay will work fine with any light so you can switch between LED and incandescent at will once you pop one in.
In any case, be very careful with LED turn signals. Most are useless for signalization and can cost you your hide. As a general rule of thumb, any LED under 3W is not bright enough.