I’ve done this a couple of times, they basically must do the following:

• return the miles first (“downgrade you”)
• rebook you to your new flight on your original booked class
• Use the miles from your account (upgrade you) again

to be transparent I’ve never been in that same situation you’re in; however from my experience, returning the miles tends to give them a different expiration date (not sure if it’s by design or just a glitch). I never had to count on it though, so I never gave it much thought.

best thing to do is contact their call centre and see what they tell you.

best of luck!