So I think there are two scenarios involved:

If R space is available on the flights in questions: Should be absolutely do-able. Call UA, ask to remove GPUs from reservation, ask them to apply yours. Choose seats again.

If R space is not available: See the posts above. SBM12 talks about it. It can be challenging and might not even possible. I would HUCA and call 2/3 times and make sure you're getting a consistent answer (the answer of it can't be done if you want the confirmed R to be saved during the swap).

Also, as an FYI, around October/November, if you re-qualify for 1K for the next calendar year, United has been extending the expiration of 2GPUs for 1K members till next April. So you'll have 4 more months to try and use your instruments!