Frankly, Delphi IDE lets you do it, and it is the most convenient way of achieveing fine editing, like changing a control from TLabel to TStaticText, moving set of controls to different parent and similar things.
Delphi, however, explicitly has two ways of rendering a for, either designer or source
Lazarus chosen a different way, to have both designer and editable source, and when i edit the LFM file - it is open editable, so why not? - LAzarus starts throwing warnings about "file changed on disk" and whatever i choose firther - it misbehaves.
This is really limiting and confusing. If Lazarus can't stand editing LFM of live form - it should had set it read-only.