The relevant bit of the new page says:
If your new ticket is more expensive than your original, you will be charged the difference. If your new ticket is cheaper than your original, we will refund you the difference. Please see our terms and conditions for details
The more detailed explanation tells you they actually permit a refund of the old tickets (allowed for other ticket types, but not advances) provided you have bought new ones for the same journey. That "will we allow this "check could include the relative prices too, if they wanted. However, the two money transactions are independent, so refunding more than they receive is no more difficult to do.
I guess the
FAQ▸ says otherwise because it hasn't been altered to keep up with this new policy.
The contradictions go deeper than that. If you go to terms and conditions, and follow that link (which they say goes to Trainline.com (sic), but doesn't), those terms don't allow for this new swap-to-amend process either.
Obviously these web page thingumies are just too hard for some to cope with.