Positive. Yes, it is. Better to say it's allowed and technically feasible.
However, Google will each time delete the old ad and regard the edited one as a new ad. Hence you'll lose history each time you effect changes.
To follow Lakatos, I would suggest you to create new ad copies and not edit the existing ones.
It's good from experimenting point of view because in case your new ad doesn't perform up to the level you desire, then you can easily pause it and unpause the original one. And in that case your original history will not be lost.
My 2 cents!
Re: Ad Copy
you should always be testing new ads.
as mentioned, pause the ad that you want to stop and upload new ads to test.
set the ad rotation to "rotate evenly" so that both ads get the same exposure.
here's an article to get you started: http://www.lunametrics.com/blog/2012/06/21/ad-copy-testing-for-beginners/