It's very easy and painless to open a OnePass account and then match your elite status to CO. As a UA 1P, you'll be either CO Plat or Gold, both of which are ahead of 1P when it comes to FC upgrades. You can continue to credit to UA as before.
OK, I took your suggestion and did just that. I now have linked OnePass and Mileage Plus accounts. Perhaps it will help in upgrading...