If I bought a Six Flags Magic Mountain ticket for 1 day use...?

Last year, they didn't. There was usually at special ticket upgrade price (more than $10) and you could credit the day's parking toward the $50 if you showed them your receipt from the parking lot. Bring the receipt from the parking lot in the park with you. They will not give you the credited (parking) price w/o the receipt. So chances are that with the parking lot receipt, you will pay (appx $35 + whatever the up grade price is). Save receipts** and bring in an extra copy of you receipt from the ticket that you purchased already (even if bought online).

**Just because you are in the park doesn't mean you paid for parking. People take the bus, the shuttle from hotels or park at the Wendy's ( the later get their cars towed). And just because you are in the park doesn't mean you paid for admission that day. People get discount tickets from their work, complementary or free tickets. So the argument " I am in the park, I must have paid for admission and parking" is bull.

Unless they changed it so that they can credit whatever price you paid for admission to a season pass, you will pay a little extra. Their register systems suck. Also, be prepared to surrender your receipts. They need it for their drawer or they will be short that amount of money (the price of your original ticket).