The ongoing cgi (ccgi) migration is all that is currently changing to a new server - homepages (www) still exists under Plusnet control and still contains the same files. These servers were replaced not so long ago so should have plenty of life in them yet. When end-of-life is reached I guess Plusnet will make appropriate plans and in due course communicate them to us.
It's up to you whether you want to move any files to the Hostopia ccgi, but there's no need to do so. I don't plan to move anything.