Registering as a non-taxpayer is not new. It is a reasonable thing for
many pensioners to do, particularly married women with a limited
contribution record in their own name. It is really up to the bank as
to whether you need to re-register, but I don't think any of them
require you to do so.
You must de-register if you income makes the interest taxable, or submit
a tax return.
You can also have the tax taken and then request a refund from HMRC,
although they would probably prefer not to handle large numbers of such
claims from people with otherwise simple tax affairs. If you are likely
to over some years and under others, it is probably better not to
register, as doing the refund form should be easier than a tax return;
for a start, you don't have to get it in by the end of January.