Editing an ad will lose your ad's existing CTR (& therefore affect your position).
Strictly speaking, this is not really true. It's the CTR of the keyword searched that affects the ad's position - rather than the CTR of the ad itself.
As noted in other posts, the primary downside to editing your URL (or ad copy) is that it'll pull your ad down from all partner sites until it is then reviewed and approved.
To prevent this, the work-arounds suggested by roitracker and eWhisper are excellent.
AWA
Thanks AWA. Are all ad changes reviewed at google?
Yep. Every new or edited ad is reviewed by an actual human being, shman.
Here are the essential things the reviewers are checking for:
* Are ads in line with the Editorial Guidelines, in terms of spelling, punctuation, appropriate use of symbols and abbreviations, etc? (Note: there is a link to the Editorial Guidelines at the bottom of every page in one's account.)
* Does the ad link to a site related to the product or service advertised in the ad?
* Are Display and Destination URLs in the same domain?
* Are any special offers made in the ad also reflected on the site itself?
Bottom line, the intent is to create an advertising program which is trusted by users, because the ads are known to be crisp, accurate, true, and reliable.
In other words, ads that people trust enough to click on!
AWA