Actually, I don't think breaking backwards compatibility would be too terrible here: my assumption is that :FORCE is in 99% of cases called manually, so as long as users are made aware of the change it should break very little.
That said, easiest way might be to make it
:FORCE-RECURSIVE <boolean> :FORCE-SHALLOW <boolean>
and deprecate :FORCE, but keep it as an alias for :FORCE-RECURSIVE for a year or so.
Actually, I don't think breaking backwards compatibility would be too terrible here: my assumption is that :FORCE is in 99% of cases called manually, so as long as users are made aware of the change it should break very little.
That said, easiest way might be to make it
:FORCE-RECURSIVE <boolean>
:FORCE-SHALLOW <boolean>
and deprecate :FORCE, but keep it as an alias for :FORCE-RECURSIVE for a year or so.