I don't know how typical this is, but I bought four Y tickets on NW earlier this year for travel to DSM from EWR connecting through MSP. Actually, it wasn't my intention to buy Y tickets -- I wanted first class since my grandparents are older and I wanted to make them more comfortable.

I bought the tickets on NW's website by searching for first tickets. Long story short, first class all the way out, but on the way back, thunderstorms over EWR cause NW to cancel their flight from MSP. I ask to be put on the CO flight (that was curiously not cancelled) and am told that in fact I don't have first class tickets I have Y-UP tickets and so no accommodation can be made to put us in first -- so I wind up flying coach back to EWR on tickets that cost nearly $2,000 a piece.

This might be one of the downsides to purchasing a Y fare and upgrading (even when you don't realize you're purchasing a Y fare and upgrading.)