silverknapper, many people have returned to flying after a considerable time off and have found it quite straightforward to get back up to speed.
Much of the UAS training will be in the long-term memory and will soon be reactivated.
Theoretical knowledge requirements will ensure that regulatory changes and other practices, which are different to the way they once were, are made known to the student. For an ex-UAS pilot, the exams are mandatory, but for someone who last held a PPL in, say, 1970 there is absolutely no requirement for any theoretical knowledge study except to be sufficient to answer a few oral questions during the Skill Tests and to regain the FRTOL.
jjoe, one aspect which you will find new is the utter obsession with 'Crew Resource Managament' and/or 'Threat and Error Management' which has crept its way into even light aeroplance flying in a completely disproportionate manner. We used to call it 'crew cooperation' and you would have been taught much of the subject without realising it. But the huggy-fluffies have made an absolute mountain out of a molehill and will bleat about 'CRM' or 'TEM' in circumstances which are utterly irrelevant.
In fact, it's highly likely that it's time for the huggy-fluffies and trick-cyclists to invent yet another term for what is only really basic commonsense.