I've done it, only needed to pay the small difference between tickets. It probably helped that I had a booked seat and wanted to travel on the same train, but was no problem for the booking office staff.
Just because it
has been done in the past, doesn't mean that it
should be done as a matter of course! The
correct procedure, as mentioned by Haywain, is a refund and reissue. Depending on the TOC, this
may be possible on the spot (assuming the original ticket was issued by the same TOC), or it may have to go via their Accounts Department or the original retailer. In fact, the (internal) Excess Fare procedures explicitly state that an Advance ticket is
NEVER an "appropriate" fare for calculating an Excess.
In a similar vein, when APTIS and SPORTIS ruled, you
could have been issued a First Class Cheap Day Return from Glasgow to York with a route of "Chesterfield" and a Gold Card discount. That wouldn't make it valid! (And would have resulted in the issuing location getting a "Fares Mismatch" letter from Rail Settlement Plan). Some of the modern PC-based ticket issuing systems will let you create such a ticket as well, but it generally needs a Supervisor to authorise it, and would probably produce the same result.