Once a registration is created the amount of that registration is set.  Any registration fees are set and cannot be altered without deleting the registration.

One exception to this is the case of additional donations.  If an additional donation was applied to the registration the additional donation can be reversed bypartially reversing the transaction, but only up to the amount of the additional donation.

If a discount code was applied to the registration that discount is set.  It cannot be undone later without reversing the transaction.