Today while flying from YVR-SFO I wanted to come home early, so I called the Premier Exec desk to make a same-day change. The nice woman on the phone informed me that my fare class was sold out but that the flight I wanted (a UA code share operated by Air Canada) was wide open, so I should have no problem getting on standby. When I got to the airport, the folks at the United desk told me that I was traveling in a fare class (W) with a restriction that all travel had to be on UA metal only. In other words, no code shares. The agent even remarked that "For a $120 RT ticket, the only code share you're going on is Greyhound." This eventually got worked out because the Air Canada and UA ticket staff were friendly (and I was a little pushy with them). Is this a common issue, or are there fare classes that prevent you from going on code share flights?

Also, during the period in which they weren't going to let me take the Air Canada flight, the agent checked me in four hours early for my UA flight - and in the process, stamp-canceled and kept the 1,000 mile paper cert I had used for an upgrade. As I still have a complete, unused boarding pass for the UA flight, does anyone think I have a shot at getting the MP Service Center to drop two 500 milers in my account to make up for the cert I lost?

The same-day-change privilege only applies to UA metal. You cannot use it to switch on or off an AC flight. As you have seen, you can usually stand by and sometimes even beg for an upgrade on AC.

A brief email to United should restore your upgrade certs.