Moved into new place - about two years old. Want to replace showerhead. Put big shifter on it last night and..... won't move. No hint of it turning. All that is happening is the pipe is flexing/bending behind the tiles

It is so firm you'd think it was on with loctite! But surely it is just teflon tape?
OK, what do I have to do. What is the trick? I was thinking of differential expansion - have the head full of cold water then pour hot water over the outside of the head? What about trying to get some WD40 into the threads?
SWMBO is losing confidence in my handyman skills!
Many thanks in advance.