[Walkman]

Then you don't need the ID at all :) because you use the slug

[eli]

I want the slug to be able to change and I'd prefer not to have to keep track of every variation ever assigned to that piece of content.

[Walkman]

if you go with /id/slug, you can redirect anything that is not exactly the same as the current, so any older links would still work because no matter what the slug was, you can redirect because the ID doesn't change.

[eli]

right, same as /slug/id

[Walkman]

no :D /id/slug is safer