Guide Changers are special machines that let you switch between the four guides, they are only available in the past so you may not change your guide in the future, Computress is the only guide in the future.
The Guide NPCs themselves also act as Guide Changers. There is a "Guide Changer" button above the "Close" button when talking to them.
Tired of working for Mojo? Are you and Edd just not Guide/Guidee compatible? There are four possible Guides to choose from when you travel back through time (Dexter, Mojo, Edd or Ben), and you can change your Guide at any time! Changing to a new Guide gives you:
1. New gear unique to that Guide
2. New Guide Missions
3. New character interactions
There are a couple of drawbacks to consider, however. Every time you change your Guide, it costs you Fusion Matter. If you change Guides often, the cost increases. You also can't use any item rewards you earned with any previous Guides. So if you love, love, LOVE your Morbucks armor, you should stick with Mojo Jojo! To change your Guide, check in with a Guide or look for Guide Changer Stations in major areas like the Cul-de-Sac or City Station.