Fredrick,

Nope, they generally work the same. The only big factor when moving from one series to other, which is the same as going from one part number to another, is the config bits. I don't recall doing anything special when going from the 16F series to the 18F. I've been using the 18F2525 myself for years and just recently started using some 16F's for small projects with no problems. If you do have problems, let us know and we'll do our best to help.