You are correct that the original Pal Mickey's will still work. However, the updated information they get when they come into the park is limited to the current operational information. Both the original and new Pal Mickeys are updated inside the parks with the current attraction wait times, show times, character locations etc. However, the original toys will not provide any information about the new attractions built since 2003.
The new toy is supposed to have over 25% new content, 3 new games, sing-a-long songs and much better jokes. He comes with a carrying lanyard and the speech volume is much louder so you can hear him better inside the parks. Well worth the price.