The flute should be the easiest instrument to handle with dentures. The embouchere for flute is either passive or actually holding the dentures in place while playing. Unlike many other instruments, most brass, clarints, etc that would be much more difficult to negotiate. So, the long and short of it, if you can avoid getting an upper denture you should try very hard not to have this as your only option, but if it happens, you should be fine with flute playing. I completely agree with Brucie though, do not get in a situation where you need a lower denture. The lower denture is at best a poor substitute for even a few natural teeth.