My experiences have been all good in this regard.
Any upgrades of fare (saver to PE) I've made (through the call centre) have been done without a change fee (although last time I was told they couldn't do that too many more times as I was a "serial offender"
). A CC fee has only been added when additional fares are payable because I always use the Poli whatsit system to originally pay, so no CC fee paid at all up until then. I've been told if I had paid with CC originally, the fee would have been waived for the additional fare due. That payment system means I save a small fortune in CC fees over the years.
The only credits I have had were due to DJ issues (or at least part DJ issues) and they have always applied the on-line rates to bookings for that credit (without me asking BTW) and without further fees.
When I've mucked up the booking (once with the wrong dates, once with the departure port and destination port around the wrong way), I've phoned immediately and they've changed without a fee, but I got the impression this is because it was done immediately not a day or so later (pays to check your booking straight away).
But.......I've also learnt that if I need to change a cheapy flight (happened probably 6 times last year for me), I just forfeit the fare entirely. I don't worry about trying to salvage any of it as the fees would often outweight the fare. I just accept this as part and parcel of using the cheapest fare buckets for business travel rather than the flexible fares. I do know that I'm way ahead financially by doing that and often if the flight alteration is due to a client, I can pass on the new booking costs to them anyway.
All in all, I'm happy enough with the way the fees are charged, and the fact it seems the booking staff do have a bit of latitude in the application of those fees. I think it pays also to have status in that regard. All of what I mentioned previously happened whilst I was silver status, but my status is accrued by lots of cheaper flights rather then fewer, more expensive flights and I have noticed the attitude of bookings staff changes somewhat when they look at my flying history and twice I got a "Oh, I see you
do fly regularly with us" before miraculously my request is granted! I've found it pays to ask for fee leniency, but I don't demand it and accept that from time to time it will cost me to make changes.