Maybe not but in my experience it's kind of tricky to take out the original bushings out the beam even when it's off the car so I'd say, yes, the entire beam needs to come off.
When you fit the beam with PU bushes the bushes get compressed. That's where they get their rigidity from. That's also why it's so hard to get the bolt to catch in the chassis. Last time I fitted a polybushed beam I had to use a trolleyjack to push the beam to the car so we could get the big bolt catch the nut in the chassis.