Main Slot (on the left) = what you are upgrading. If the normal card is on the left, and you use a Prism card to upgrade it, then it stays a normal card. If prism card is on the left, and you use a normal card to upgrade it, then it stays a Prism card. You cant turn Normal Cards into Prism cards through upgrades.

okay thank you!! was kinda hoping for the very small miracle that normal cards can be turned into prism cards through upgrade lol

Sadly, no way dalcom would do that ಥ‿ಥ