It doesn't appear that the preslope is under that liner, so to do it right, it needs to come out as well.
Proper buildup on a plywood subfloor is metal lath stapled to the floor, preslope, liner, setting bed, then finished tile floor. Check out www.johnbridge.com for help with that part of the construction. If I was going to do this, I'd use the Kerdi system from www.schluter.com. That would require a different drain altogether. They have some videos on the Schluter site to see how to do that. For custom sizes, you'd still make the preslope almost the same, but that becomes the only layer of deck mud needed...then you apply the Kerdi membrane and tile to it.
It somewhat depends on how long the riser to that drain is. If it is long enough, you could use an internal cutter, cut the pipe, then use a coupler to add a section back and a new drain assembly. If you can get the top part of the clamping drain off, you might be able to use a hack saw blade to cut through the outside of the drain assembly and peel it off. You'll possibly mess the pipe, but often you can with some finese.
If you want to stick with a conventional shower construction, you may need to buy a new liner if you can't get the drain out without cutting.
Keep in mind that the deck mud underneath the liner at the drain should be about minimum 3/4" thick (you could get by with less if it was a concrete slab), otherwise, it doesn't have enough strength. It looks like yours is installed flush with the floor, so it'd be too low with a proper preslope then liner install. Hard to be sure from a picture, though.