I also need to change dates on an upcoming flight, and noticed that the "priority + 2 bags" that I paid £10 for on the original flight, now comes up with an additional £10 charge as price difference for "priority + 2 bags" if I want to make this change.

If I try make the new date's booking as a fresh booking, the charge for "priority + 2 bags" is £12 (base fare is the same for change vs fresh) so if anything I expected to pay £2 extra, but here they want to bump it up to the max £20 they can charge in total... T&Cs keep it vague for changes, so perhaps in some twisted way it allows Ryanair to do this.

If you remove the “priority + 2 bags” when making the date change, I think you might end up discarding it and lose your original £10 paid, and if you then want to re-add after confirming the change, you’d pay the full £20 post-booking charge as extra. Whereas if you pay the price difference I’d pay £10 extra.

OP - in your case, it’s much higher values at stake so I’d recommend confirming the implications in writing from a Ryanair agent before going ahead with anything.