I requested a PC after my Diamond Preferred card passed the one year mark. It was easy and didn't take too long with a regular CSR. I wanted to PC it to a Dividends Platinum Select card but apparently I couldn't PC it to that, but instead it will become just a plain "Dividends" card, which does not show up on the Citi website's list of cards but apparently has all the same features/benefits. Not only will the new card exist under the same tradeline, but the account number stays the same too (the CSR seemed surprised about that but verified it with a supervisor).
Only downside is that I have to wait something like 45 days before the PC actually takes place, so that I can change my mind if I want in the interim, probably a result of the Credit Card Act. Oh well, can't wait to finally get rid of this stupid Diamond Preferred card!