Not sure where to post this but first time problem for me

Is it true that FB awards cannot be changed but instead you need to cancel and rebook the entire trip? This is what the PSL rep told me and this being my first award trip caught me unaware

I am booked on a EU-CDG-EU flight the last flight I wanted to change to the earlier flight that now has award space but since I bought SAF for the journey I cannot cancel or I would lose the SAF money paid

Seems rather clunky if only cancelling works?