There is no difference in price for a FTTH service to any copper service like for like. It is only if you opt for the higher speeds that there is a price increase.
There is currently no forced migration from ADSL/VDSL to FTTP , but if an area achieves the appropriate level of OR FTTP penetration ( 75% I believe ) then there can be a copper stop sell applied by Openreach, so anyone moving provider on Openreach , re contracting with their current one , or taking a new OR based service , it’s a new FTTP installation,( or moved onto FTTP) as part of that order process ( remain out of contract , nothing changes ) , but as already stated , there is no price differential between FTTC and FTTP , so taking someone on an 80Mb profile on FTTC , is no more expensive to move to FTTP 80Mb , so why not switch ?, there is no compulsion to take the higher speeds FTTP can deliver , if happy with 40Mb , stay with 40Mb .
In many cases OR incentivise the ISP to move their customers onto FTTP when it becomes available ( even below the 75% threshold ) , so as well as no increase in monthly cost , most will also provide the FTTP installation for ‘free’