I don't think muscle turns into fat. I mean yes, you can get fatty striations in your muscle but you're more likely to catabolize your muscle, burn it for fuel, rather than have it turn into pure fat.

Muscle also helps to support your bones which is a good thing for arthritis. You want muscle to be doing it's work so there isn't so much wear and tear on the bones.

Besides, most of us don't really build muscle all that quickly or easily.