GeckoFriend - the only way you can "mess up" your books using Sigil is either if you were to attempt some sort of "replace all" (or you fall victim to one of Sigil's various bugs).
My experience of the <address> tag has been that trying to come up with a generic find/replace regular expression for it is not possible, since it could be at the beginning, middle or end of a sentence, and could sometimes denote the end of the paragraph. So long as you just simply step through the find results and manually edit the document like I mentioned above then you will be perfectly fine.
At the end of the day it is not a life or death change to make to your books - if it scares you then don't do it
. However I get annoyed with seeing a sentence broken onto a new line when the reading device happens to hit the <address tag and puts the word in a different font size etc - it just breaks my reading flow which irritates. So if you have OCD tendencies like me, this way you can find & fix them before the book hits your ereader
Anyways, glad you are now sorted.