upgrading to FLRes passes???

[disney momma]

i'm booking a free dining package, which requires us to buy the regular MYW tix. After we have used those (total=$442 for me and DH), can we upgrade to the FL res. passholder tix? what if the regular tix for 3 days total more than the passes (FL res. seasonal passes, total=$430 for me and DH). can we basically just trade them in for the seasonla passes??? i'm confused about the upgrading system...