Yes, you can change the id if you really want to, but you'll be going off the beaten path there, so it's quite a bad idea. Let Geddy handle IDs for you.
The way I would do this (and certainly how many others have too) is to have a "slugging" function create a slug from the post title, and save that in your database. Then, query on that instead in your show action. You won't have to change your routes.
This is what your query will look like in the show action:
Post.first({slug: params.id}, function (err, post) {
params.id
is whatever string you use in the route /posts/<this string>
So once you change your show
links to use the slug instead of the ID you will be all set!