Amending a reward seat booking

I have booked return flights next summer using points and upgrade vouchers. My question is - if I change both my outbound & inbound flights at the same time is that just one £30 change fee?
I know I could cancel flight and rebook but I am concerned about losing the use of my upgrade vouchers as they may have timed out . Any help /thoughts would be appreciated. Thank you
I know I could cancel flight and rebook but I am concerned about losing the use of my upgrade vouchers as they may have timed out . Any help /thoughts would be appreciated. Thank you