So I think you can only redeem the rewards if you have ZERO boosts - so you need to use them all up and hope that then you can convert! That's what I read from the info in the Image Creator Help:
"6How do Microsoft Rewards work with Image Creator?
If you run out of boosts, you have the option to use Microsoft Rewards to redeem for additional boosts and enjoy faster processing times. When you run out of boosts in Image Creator, you'll also be reminded that you have the option to
redeem Microsoft Rewards points for more boosts."
So it seems it won't offer to let you "buy" boosts until you've used up yours.. so a bit risky as it doesn't even say how many boosts your rewards will buy and if you use them all up in the middle of a creative session and you can't buy enough boosts with your rewards to finish what you're working on, you may just end up having to wait a week or so to build up more boosts!