simple, albeit stupid question.
 

I have a part which is tapped M6. The bolts I have are 1/4-20

M6 is .236" dia

Is it possible/advisable to retap the 6mm holes to suit my bolts?

I would prefer not ordering new bolts, but I especially don't want to mess up my part. If I had an M6 tap, I could just do some experimentation on some scrap. Sadly, I don't.

My gut feel is to order new bolts, but I want to check with the brain trust first.

Personally, I would go with your gut and order new bolts. I don't think there is enough 'meat' left to re-tap and get full depth threads. But someone with more knowledge than I must answer that question for sure.

The drill size is almost exact (.1mm off), it's the pitch, that's not close enough, 20 tpi compared to 1.0 = 25.4 tpi

You could get away with re-tapping, if the holes are deep and a lot of threads will be engaged, and the material is steel and not alu., but it is still not a nice thing to do.