I went ahead with my change and indeed an agent confirmed that Ryanair systems follow the unwritten policy of "upgrading" the baggage charges to the full after-booking rates, even though you are merely maintaining the same baggage options during a date change. They don't consider the price difference vs a fresh booking, since the baggage rates on a fresh booking are discounted. The agent couldn't comment on why they do this, but it is what it is.
[MENTION=10075405]Jacotey[/MENTION] - this means if you discarded your baggage options when making the date change, my guess is that you may have effectively "lost" the value (presumably £64) that you had already paid for these options, and to re-instate all the baggage options, you may end up paying the full after-booking rates (presumably total £128). To pay the price difference of £64 at the time of making the change would have been the less costly option. Maybe you could ask Ryanair to make an exception and help you recover and use the "lost" value when re-instating the baggage options?