Is it okay to put 3/4" plywood over 1/2" wood subfloor?
There are no structural issues with putting extra subfloor on existing subfloor.  The only issue this would cause would be matching of the floor heights and possibly your first stair height.

(and trim - subfloor could make you reinstall/cut all trim)

(and make sure the old subfloor is even/flat or patch it first)