DIY bathroom floor waste

Well I have now gutted my bathroom and adjacent toilet (spent about 5 hours on the end of a jackhammer digging up the old tiles and floors!!) I would now like to replace the old white, octagonal, pvc floor wastes with new round, metal wastes before the tiler comes in and does the floor.

Looking at the existing wastes, they appear to be held in with a motar mix similar to the sloped floor bed. Should I have a go and replace these myself or is this best left up to the tiler or plumber? I haven't priced new floor wastes yet, but what are the each type roughly worth?

How would I stop all the rubble from falling down into the pipe when removing the old wastes? How hard is it to cut the tiles to suit a round waste? I would think that tilers prefer the octagonal wastes as they would be easier to tile around???

To prevent rubble falling in, just get two or three plastic bags of the supermarket variety, put them inside each other to make one loose bundle and just pop them in the top of the pipe, so that the plastic is sticking out a bit. As you go, remove any bit chunks that might fall in and push the bags down.

When you have finished the job, just remove the bundle gently and it will bring out the bits with it.