If the drain is in the right location, there's no reason to replace the p-trap.
You will need a clamping drain for the mud set pan, and after that is installed, you will need to pre-slope the base, which will include filling in the hole.
With a standard enclousure or fiberglass pan, the box out is fine and how it's done. Not so fine with mud set. The concrete mud based pan will be the support.