while it can be a case of YMMV - it is more the norm rather than the exception that a change of class - even where the higher miles have been paid (but a member accepted a lower class at the time of booking), will attract the change fee.
i was lucky enough to get a flight changed - just the date only... same flight number and route without a change fee
she told me "you understand it is $150 per person per change" i said "yep no problem at all" then we went through the changes and she forgot to charge me. I never received a confirmation of the change via email but check my itin and sure enough the date had changed.