Interesting enough there is a "toggle case of selection" feature, but there isn't a "toggle quotes of selection". I'd find it very useful in languages as Perl, Bash, Python and Javascript where you can write strings using `'` or `"` characters, but they have some semantic differences or you want to have a consistent way of quoting strings along all your source code.
The feature would very simple, select some text, find first quote symbol, replace it by the opposite, look for the corresponding quote and replace it alike.
This feature is available in Atom text editor through an external package: https://github.com/atom/toggle-quotes, however, I thought that in Geany it could be included in core as other toggle features are in: "toggle case of selection" or "toggle line commentation".