As far as I can tell the free line rental stops being free so the cost goes from £29 to £59 a month.
So it comes down to 'do you need to maintain the copper circuit at all', eg for the VDSL FTTC connection?
Do you have FTTP options available?
If you do need to maintain the existing copper line, then look around and decide. Least hassle is probably to just sign up again, but there may be better deals. But possibly with enhanced crapness.
If FTTP is available, then you can ditch the copper line and line rental totally.
If so, then I would go with that, with an ISP of your choice.
If maintaining a landline number is important (it was for me), then that's a separate thing and you would migrate your number to a VoIP provider, get an IP phone, and use that provider.
In my case, my VoIP provider is also my ISP, but there's no reason it needs to be.
If this is all a bit too much and making your brane hurt, then the path-of-lest-resistance would be to just sign up again...