To the best my my knowledge, once a Mileage Plus account is associated with a Mypoints account, there's no way to un-attach it. Even if you choose to close the Mypoints account, their system doesn't allow you to "recycle" the once-registered Mileage Plus account to another Mypoints account.
That's also why you can't redeem from multiple Mypoints accounts to one UA account. Read the recent discussion
here.
Originally Posted by
Marathon Man
You are seeking what, about 1,000UA miles?
FYI. There's no 1000 UA miles increment redemption from Mypoints. You can only choose between 2500 miles (5K points) or 5000 miles (10K points) increments.