Replacing tiles currently laid on ply wood

[Toots23]

Waterproofing a small shower room in a loft conversion. Original tiles laid on ply wood. Solutions offered are: lay ditromat (but there is not much space under existing tiles and may raise the floor too much in relation to shower tray. Or replace ply wood with hardi board.
Any views?

 

[bsc ceramics]

Replace the ply with hardi and screw down every 6ins