How can I convert tabs to spaces and vice versa in an existing file

Since fix of: https://github.com/Microsoft/vscode/issues/1228 the editor supports it out of the box. Simply go for:

1. F1,
2. indentationToSpaces or indentationToTabs (depending on your need)
3. Enter.

Another way to do it is click the current indentation (Tab/Spaces:n) on the footer which will open your indentation options where you can select what you want to do.