There is little Whonix specificness here. It's similar to using many torified applications on the host, then changing circuit (using Tor Browser's New Identity feature or
arm) and asking "are circuits changed in all applications or only in Tor Browser?"
Tor has no concept of "workstations" (neutral statement). At best it knowns that it's getting connections from separate client IPs (neutral statement).
IsolateClientAddr default stream isolation applies so or so. From Tor manual.
Don’t share circuits with streams from a different client address. (On by default and strongly recommended; you can disable it with NoIsolateClientAddr.) Established connections keep their circuit. New connections will use fresh circuits.
So the answer is a bit more complex.
Circuits of long running connections (such as a long download or long IRC session) will not change their circuit - no matter from which workstation their are originating.
New connections however will use fresh circuits - no matter from which workstation their are originating.