How much is it acceptable to partially answer a post within an edit of that post?

I have encountered a situation in which the OP asked a relatively simple question about manipulating logarithms. The OP was clearly not practiced in the subject, and asked a question involving the simplification of $\log{x}/\log{10}$. I answered the question soon after it was asked. Someone then edited the post for clarity, but in doing so, changed the posting to $\log_{10}{x}$.

I asked the editor of the post if it was OK to partially answer the question in the edit. To be honest, I don't feel very good about it, because it takes away some of the content of the OP's question and robs other readers of finding out what the OP needed to know. That said, I would like to ask the community if there is a policy on edits in that direction.

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I feel this sort of edit should be rolled back, since it changes the content of the question. E.g., the question "why $\sin^2 x +\cos^2 x=1$ ?" is not the same as "why $1=1$?". –  user53153 Feb 18 '13 at 20:02
OK, good, I'm not the only one who feels this way. –  Ron Gordon Feb 18 '13 at 20:07

What?$\vphantom{}$ –  Asaf Karagila Feb 21 '13 at 0:48