It all depends on your plumbing skills, push fit isnt difficult, just get the basics right. The easiest (not the best) way would be to cut away a little more around the pipes in the floor - unless the pipe can be lifted up slightly. You want to raise the T connection above the floor so you can not only remove it but grip the pipe otherwise if you let go the pipe will spring back under the floor and any water in the pipe will leak out. So you want to be able to hold the pipe remove the T connection and then push on an elbow connection - make sure the inserts are in the plastic pipes. Then run across the floor into the wall and likely another elbow to have the feed go up the wall. You could cut a small letter box size window around the pipes from the floor, shouldnt impact on floor rigidity but enough to get you hand in and see what your dealing with under the floor and how much play you have in the pipes.
Some points - make sure the connection for the shower fits your new one, and you have it the right depth depending on any wall tiles added over. - -personally I would replace with a new connection coming out of the wall for the valve bar to be fitted. most are 150mm centres. You can get plates so guaranteed size or just measure and secure your own - whatever you do thing several steps ahead - you dont want to be sticking something in the wall and then after tiling etc realising you've made an error - so have all the fittings checked out for compatibility and get it right from the start.
What I would probably do - remove all the pipework down to the floor. Im guessing all your pipe runs are plastic with push fit. I would install a 4x2 beam between the middle of the shower bar up the wall so you have an easy option to screw the riser rail to - much sturdier than using plugs. I would add a couple of horizontal beams down under the shower bar -to clip the pipes up the wall to rather than have them hanging or bent out to the side. I would then run copper pipe up the full length and solder copper elbows to come out from the wall with copper pipe then run across the floor to the connections. If you have copper under the floor I would run it all in copper. I would have two copper pipes running out for the valve to be fitted and then fit using something like swirl bar valve connectors from screwfix. Having the copper pipe come out of the wall means you dont need to 1. use those stupid offset connectors that come with showers, and 2. dont need to worry about wall tile surface depth - just leave 4 inches protruding, cap them off and when all the walls done use a pipe cutter to cut to the right length fit the swirl connections.
