Do I need to replace moisture resistant plasterboard?

I’m renovating my bathroom and installing tile backer board on several walls where tiles were removed. These walls are in and around the shower.

There are two other adjoining walls with moisture resistant plasterboard. Should I remove the plasterboard from these walls as well, or can I leave it and tile over it?

If I were to keep MR plasterboard, would I need to sand down the paint before sticking tiles to them?

The tiles I’m using are large 1200 x 600 x 8mm.
 
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