I have the same question! Surely I have seen previous topics on this issue, but none of those were dated less than 2 years ago. I was wandering wether my existing links will still break if I modify my permalink structure, on the latest version of WordPress (3.1.1.)
Would enabling Fancy Permalinks, per se, break my existing links? If so, can this be avoided?
I don’t think that if I try and change my permalink structure and then change it back, my posts (and therefore links) would return to the same ID number. I would need to be sure of that to be able to try it.