in my experience, having done this lately, if within one year you decide to return to Europe and the fare bucket letter is available as it is written on the ticket, an agent will process this with an even exchange. I’ve had this happen 3x since the change fees were ‘waived’ and not had a single problem with an EXP phone agent doing this for discounted I-fare business class tickets.

You have to read the fare rules in depth. The fare rules usually state something like “travel must be completed within 330 days of commencing” or something Iike that. You just need same fare class availability on a future flight to change it.