As long as you 301 the old links to the new links they your rankings / page rank etc would not be lost. It may take an update or two (of the page rank toolbar data rather than the 'real' data) to show the changes, but I have done this many times in recent years, as recently as 3 weeks ago, and no rankings have been lost, often they slightly improve as it freshens things up and makes the links more 'relevant' feeling.
But, if you get it wrong / don't redirect with 301 header codes then you are looking at some serious issues.
In your particular case, I would agree that saving this change for when you see things starting to sink would be wiser than messing with what works. If it aint broke....
As an aside, clean links can help users slightly with reference, sending in emails, linking back to you, bookmarks etc, but not enough to make that the reason to do it
