I am 100% sure a 61.75 is valid only if the underlying license is valid at the time.
There is a grey area regarding type or class ratings. The usual example is a PA46 which needs a TR (or a CR?) if flown on a CAA license but not if flown on an FAA license. In that case, it isn't clear whether a 61.75 is valid if the UK PPL doesn't have a PA46 TR/CR on it. That one has been argued both ways.
What a lot of people also get wrong is that they get a 61.75 and don't get a US BFR.