In a case such as this you need to buy fittings to suit the holes .
Or, I recently did a job where very large , olde fashioned recessed lights came out ., drilled a new hole next to the old and got a plasterer in to patch.
:slapProbably filled all the new holes in and left a rough hole where the old ones were.
Unless the exisitng hole is a misshaped messif your making the hole bigger, then use a holesaw the same size as existing hole as a pilot. no need for a hole in plywood