Can not see the point about the new shower being the same out put as the old so maybe ok to just swop over..you do not know if it was installed correctly anyway,just because it has been working 30 years..part p reg or not..if you are working in someones property,you should at least have a basic knowledge of what you are doing and you should not be putting customers at risk while to do so...you have proven by the posts you are not confident as regards the shower update,so I would say step aside and let someone do the the job that can do it correctly
A big part of our job is to know our limitations,pass on work we can not do and thus cover your bum.......