I too switched from DL Platinum to CO this year. Since CO & DL are partners you will have to do an extra step. I got a status match on UA to their mid level then used that to get a status match on CO at Gold Elite. You drop one level but if you fly a lot then you will regain Platinum on CO. At least this worked for me this year.

I fly mostly international in C or F and I think that CO 's biz product is much better than DL. At least until DL gets the new angled flat seats in 2008...then I will have to rethink.

For domestic flights on CO this year the few routes I use are sometimes on an RJ so no upgrading necessary. I had 1 round trip from EWR-MSP and was upgraded on both legs as a Gold. The other flights I purchased first class tickets for EWR-LAX & EWR-LAS. Price is not too bad and the service is very nice. Especially on the transcon flights.