It is important to note that the change must be done prior to departure.
If it's zero cost with GWR and you don't think you'll know when you need to travel prior to the date/time of the original tickets, I'd recommend moving them to a date near where you think you'll travel (preferably to a service which costs as close to the original value of the tickets as possible) and then change them again when you're sure of your travel times.
If you move a £60 Advance ticket to a service that costs only £30 and then try to change the ticket again, you'll only get (or should only get!) £30 towards the second round of new tickets.
I'm just going by my recollection here, but with the Trainline's implementation of changing Advance tickets (which also applies to 'white label' TOC sites such as Crosscountry), if the replacement Advance ticket would otherwise cost less than the original then actually in the 'change ticket' interface it just shows up as the same price as the original ticket - you can see this when you compare (a) being logged in and searching to change a ticket and (b) just doing a normal search for tickets (using say a private browser window if needed, though I don't think this is actually necessary).
I don't know whether the Trainline actually fulfils the new ticket by reserving one from a more expensive (presumably equivalent if available) quota allocation - though I suspect it does that - or if it (or the industry at large) just pockets the difference.
Apologies if my description of the above is confusing - the correct (or at least more accurate / clearer) terminology currently evades me!
Crosscountry's
'no admin fee' for changes to Advance tickets can be quite a useful facility.
It should be noted that XC does state that...
You can make changes to your Advance train ticket 2 hours after you have purchased and up to the day before the departure time on your ticket.
However in practice I've found it's possible right up to just before the scheduled departure time of the booked train... and I'm not sure about the 2 hour after initial purchase thing either. But it's worth mentioning just in case their implementation of this has changed recently!