You're in kind of a tough place. If you reach 75k, you'll be Platinum with CO next year. That would be a great level if you were actually flying UA/CO...

If you switch over to US and start from scratch, then you lose the mile earning bonus until you catch back up.

I wonder if you could do something like this:

- Earn 75k with CO to achieve Platinum
- Status match to DL or AA
- Status match back to US

No idea if that's at all possible...