Straight swaps can often have a change of riser rail length, manufactures may keep the same base size and guts for models from one year to the next. But occasionally they change the dimensions of the rail, straight vs curved, long vs short, new molded wall brackets or something else. This can leave a bit of re-drilling and patching old screw holes. Bit of silicon or ready mixed grout helps with this. While the old shower is off I would suggest a full set of tests on the cable while you have some open ends to work on. I would think you need to allow 1/2 a day to leave yourself a comfortable unrushed job.
Doc H.