I agree with FD. Going back to G now rather than if/when required, doesn't add up.
The exceptions might be
1. The owner (an FAA PPL but not yet an FAA IR) absolutely cannot enter U.S. territory for some reason (I can think of a few, not suggesting of course anyone here has such reasons!). Thus, if FAA checkrides were to become impossible outside the USA, he has no way to obtain an FAA license/rating. BUT remember that the CAA/DfT has no means to prevent FAA licenses being obtained elsewhere outside the UK, nor can they ever prevent remunerated training taking place outside the UK FIR. So this paragraph really applies to the FAA banning checkrides outside the USA. I've seen no sign of this as yet.
2. The owner will be unable to revert to G in the future but can do so now. I can't think of a plausible scenario for this. Maybe there is a lifed item (hourly-based), perhaps an airframe hours issue, which enables a G CofA to be issued now but will prevent it being issued after X hours have been reached, without a lot of expensive work.
3. One's maintenance situation makes FAA work much harder to get done. For example one might have hangarage with a JAR maintenance shop which doesn't have an FAA approval (and refuses to get an FAA IA to visit and sign off the work) and which absolutely insists on maintenance work as a condition of the hangarage otherwise they will kick you out of the hangar.
Otherwise, staying on N now and going to G if necessary sometime in the future is the least cost option.
As I've said before, there is no such thing as peace of mind in GA.