As far as cracking plaster.. who the heck knows?... Don't touch it and you shouldn't have a problem. I did a similar job several years ago. If you have an 8' space you should be able to squeeze in a tub/shower and a toilet. If you have PVC then it's relatively simple to cut out the sanitary tee and flange then install a "y" or tee on its side to route to the new toilet location. There's a difference between a "coupling" and a "repair coupling" the repair coupling doesn't have a stop in the center and can slide up a pipe and out of the way while a replacement section is put in place then you can slide the coupling over the new pipe ends... helps alot when remodeling and moving toilets. Biggest problem I've run into is having to place a toilet where a floor joist runs which calls for boxing off the ends of the floor joist that has to be cut and sometimes having to patch the floor to cover the big hole made to get to the floor joist.