Originally posted by ozstamps:
Raddy .. welcome to FT!
I don't quite recall how it works if UA does take C points out but can't get you there both ways in C, but only Y for one of the legs.
Why you want to go one way Y and the other C beats me, but that was your question. 
I seem to recall they adjust the mileage deduction if THEY could not confirm the C when you flew? Someone here will know.
Anyway, if that is the case, flying the route you WANT to be in Y by choosing a madly overbooked flight in C with lots on waitlist for upgrade (Friday would be the perfect day!) might get you what you are seeking?
As far as I know, if they cannot confirm you in C both ways but only one way, they will deduct the C class miles from your account and waitlist you for C on your confirmed Y segment.
If the C class doesn't clear, I don't think you get any miles back unless it is a special case.