Let me point something out that I've neglected to mention:
once google crawls all of your pages, everything should go back to normal. And you may see better rankings
No matter what, when you change the URL and/or keywords on a page, your ranking will change. But that means that your rank (SERP) can go up... or it can go down.
To make sure that you're making wise changes, I would suggest checking out your pages and seeing what they rank for certain keywords. Google certain keywords and see what position your page comes up in (there's tools out there that will check SERP for you - Google 'check serp' or 'check keyword ranking') and make note of the search term and search engine ranking.
SEs can take some time to index all pages, so I would make your changes and wait a couple weeks then check your pages again for the same keywords. Now you have a record of your old ranking position and your new ones. Then you have a concrete measurement of how effective your changes have been.
I hope that made sense...