Are you sure you want to switch?
Honestly, unless you have a really good reason to change to UUIDs (CHAR(36)), then I would recommend just staying with auto-incrementing IDs. There are plenty of people that tout the benefits of each, but it boils down to auto-incrementing IDs can be faster, and unless you have multiple databases where you're worried about overlapping data, auto-ids are just fine. (And it's not a simple "switch")
Not crazy-simple:
If you still are sure you want to switch to UUIDs, there is no automated process, but be careful - it's not just about switching the field types and voila - you'll have to create a script or something to update the id fields as well as all the associated fields (ie 'user_id' in the 'actions' table won't be updated..etc etc).
If so, here's how:
So - create a duplicate of your database (or tables) as back-up. You'll then probably want to rename the 'id' field to 'autoid', create another id field CHAR(36), run a script to populate all the UUIDs, then another script that populates the associated ids (ie 'user_id' in the 'actions' table) with the corresponding UUID.
CakePHP code that generates UUIDs:
Here's the link to creating a UUID in CakePHP 1.2: http://book.cakephp.org/1.2/en/view/826/uuid