user179: Mainly when I answer a question I tent to think about It for quite a while, often coming up with some addition to the already given answer. 
Is it alright to simply add it to the existing answer or do you have to mention such changes in case it changes the nature of the answer as whole?

user408: Expanding on an earlier given answer is fine and IMHO should be encouraged. People can always look at the edit history so they can see what has changed. Also when you edit an answer, any upvotes on it (which are normally locked after voting) can be undone by the voter in case he/she now disagree with your answer.
Some people prefer to explicitly mention their edits in their answer text, for example by adding the text "EDIT" at the end of the original text and then add new text (see for example this question or this answer), but this isn't mandatory. Personally I only do this if the edit is substantial (but still related to the original answer).

user143: I spent some time on the Meta FAQ page FAQ for Stack Exchange sites to see if they had anything about an original author explaining changes to their own posts and I didn't see anything specifically about that. I suppose if you were changing your own answer so drastically that you didn't feel it was proper to keep the upvotes your other answer had received, maybe you could delete the original answer and submit the new one to see how it gets voted on it's own merits. But for minor enhancements I didn't see anything. And certainly on this site, many answers show that they've been edited at one time or another. 
Here was one FAQ that went into detail about when it's good to edit both your own and others posts as well. Why can any user edit any other user's question or answer?  Editing seems to be encouraged overall. 

179: <p>Mainly when I answer a question I tent to think about It for quite a while, often coming up with some addition to the already given answer. </p>

<p>Is it alright to simply add it to the existing answer or do you have to mention such changes in case it changes the nature of the answer as whole?</p>
 408: <p>Expanding on an earlier given answer is fine and IMHO should be encouraged. People can always look at the edit history so they can see what has changed. Also when you edit an answer, any upvotes on it (which are normally locked after voting) can be undone by the voter in case he/she now disagree with your answer.</p>

<p>Some people prefer to explicitly mention their edits in their answer text, for example by adding the text "EDIT" at the end of the original text and then add new text (see for example <a href="https://buddhism.stackexchange.com/q/1348/408">this question</a> or <a href="https://buddhism.stackexchange.com/a/35/408">this answer</a>), but this isn't mandatory. Personally I only do this if the edit is substantial (but still related to the original answer).</p>
 143: <p>I spent some time on the Meta FAQ page <a href="https://meta.stackexchange.com/questions/7931/faq-for-stack-exchange-sites">FAQ for Stack Exchange sites</a> to see if they had anything about an original author explaining changes to their own posts and I didn't see anything specifically about that. I suppose if you were changing your own answer so drastically that you didn't feel it was proper to keep the upvotes your other answer had received, maybe you could delete the original answer and submit the new one to see how it gets voted on it's own merits. But for minor enhancements I didn't see anything. And certainly on this site, many answers show that they've been edited at one time or another. </p>

<p>Here was one FAQ that went into detail about when it's good to edit both your own and others posts as well. <a href="https://meta.stackexchange.com/questions/120576/why-can-any-user-edit-any-other-users-question-or-answer">Why can any user edit any other user&#39;s question or answer?</a>  Editing seems to be encouraged overall. </p>
