I am with others who say buy Z fare. I do so for my personal trips often too. As we all know UA (as other US based airlines) do not treat upgraded passenger the same way as a passenger who paid for a business class seat. The difference only shows up in cases of IRROPs or when one needs to change flights because of their own reasons. But then basically you are back to Y with just a possibility of an upgrade at a short notice. In case you are rerouted to another airline, even partner you have not even a possibility to be in C. If not being stuck in Y for a long trip is important to you (as it is for me) the only choice you have is to buy Z fare.

I usually use SWU either to upgrade C->F (obviously when I have at least D fare) or in cases when the price difference between W+ and C is really unacceptable. For example for a personal travel at a very short notice or without Saturday night stay (which makes it a difference between M and full C and might be $1K verse $8K).