I am quite sure that replace-by-fee doesn't allow the sender to change the amounts being broadcasted. You can change the miner fees and the receiving address, but you are still spending the same UTXO. Therefore, if the original sum was 0.0055 mBTC, it has to stay 0.0055 mBTC. D
You can change the amount by adding more UTXO(s) or removing UTXO(s).
Let's say the original transaction is as follows.
Input A -------> Output A
Input B
This transaction can be replaced by following transactions.
Input A -------> Output B (higher fee)
Input A
Input B -------> Output B (higher fee)
Input C
Input A -------> Output B (higher fee)
Input C