Who said that the number would have to change? For a simple switch of providers it will probably be *at least* as smooth as it was when you moved the number from BT to Zen because of all the extra years these organisations have of doing this every day. (Who would bother to log in to a support forum to report back on this - we only see the cases here where it doesn't work).
Moving the land line numbers between providers has been pretty transparent for my and my parents' switches between different providers over the last 10 years (including in and out of Virgin Media; from BT to Talk Talk to Sky, and including from physical phone lines to the new "Digital Voice" lines that Sky and VM use by default already).
If it doesn't "just happen" on the day then it can likely be fixed over the next days and weeks by BT.
Zen do have a reputation for a slightly higher level of service and support than other providers, and if that's worth paying for to you, or don't accept any risk then it's a sensible choice to stay with. My brother loves them after the advice he got from them to fix his setup for what he needed for his work (he creates and plays computer games).
In my recent experience, BT has been helpful and responsive around setting up the details of my recontract, and retrieving my phone number of 28 years after it has spent 13 days with Virgin Media 😄
Maybe it will be a good excuse to phone a few old friends if I do need to change it? (Probably I'll just text their mobiles though :D)
