This change (upgrade) is exactly the same (except for the BT logo dressing) as that being made compulsory for people with ordinary Yahoo email accounts. I have an ordinary Yahoo email account, which I get POP delivered. I also converted to the new Yahoo mail interface (for the occasional times I go there to check spam etc), and it's fine.
AFAIK with Yahoo mail accounts, you do need to access them or they close them down. You can usually reinstate the account, but I do believe you lose whatever was there.
So this seems to be driven my Yahoo, not BT. If you POP access the account there is no problem, though I don't know what happens if you have any contacts in the BTYahoo interface (best not to as if the account gets hacked into, they can't get to them).
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