Think about it--when you change LAX-FRA to LAX-FRA-DXB, more miles are deducted (actually US -EU is deposited, US - ME is deducted) and you only pay the change fee.

It is the exact mechanism for changing from a standard to saver - not sure what the codes are, but something like to YN52 to YB52, where the system auto-redeposits and deducts the new amount.

There is never a redeposit fee to change an existing PNR.