It would be nice if one could just select some comment block (or maybe open the Refactor menu with the cursor in it), and have the ability to convert  between line and block comments as a refactoring action available.

There is a slight ambiguity how greedy such a refactoring should be for line comments, but I'd suggest to apply it to consecutive commented lines, even if there are empty newlines in between. As in

// some other commented out line


// code placeholder

// multiple lines of
// comments<CURSOR> can
// be found here

would after selecting the refactor action

/*some other commented out line

code placeholder

multiple lines of
comments can
be found here */

Alternatively, one could require that all of the lines have to be selected before applying the.refactoring