This sort of edit tends to be obnoxious and should be discouraged.
The change in quote styles is a typography issue. This are valid use cases for them, but usually those cases are in print work or graphical design. Most on-screen typography leans towards using the normal ASCII character rather that specialized glyphs. Especially in the context of technical sites and programming, using them is usually to be discouraged in the first place. SE sites are not optimized for print and tend to be technical. An argument could be made that the change is sub-optimal to begin with.
But more than that, even if the change was valid, the annoyance and confusion of having such an edit overweights the benefit. It's confusing and annoying to most users to have their style messed with. There are enough cases where style edits should be made (e.g. fixing where people have ignored capitalization and punctuation rules entirely making their posts unreadable) that doing it where the only motivation is a style preference that might not be shared by the author just muddies the waters.
Edit: The above applies if the main focus of the edit is the quote style. In the context of edits that make substantial changes to the rest of a post I think editors can use whatever quote style they feel like. If the OP posts garbage that is grammatically broken, does not capitalize or commits other crimes against typography, that OP has forfeited the right to have a quoting style preference and any editor willing to clean up their mess should be allowed to use the quoting style of their preference.