Well, yes and no.....
As "skssgn" pointed out, the subframe is in the same location, so having either the round or square front on the same car is possible..... but the main issue would be the inner guards.
For 2 reasons.
The clubman inner guards appear much heavier due to the formed upper section. This bit basically blends the wider clubman bonnet opening to the radiator shroud.
The length of the inner guard from the bulkhead to the headlight mounting panel is a couple inches longer on the clubmans too.
These 2 reasons make the fitment of a round nose front slightly challenging.... but not impossible. The clubman inner guards can be kept, although they would not be able to support the outer guards in the correct positions, and would need modifying to suit the shorter round nose guards.
If it was totally necessary to keep the inner guards, the smart thing to do would be to have them detatchable aswell, possibly by having the inner guard overlap the body (or fabricate some 90* edges that would mate together) in the section above the upper shock brackets... held together with a few bolts or quick release fasteners..... and then making a bracket to locate the lower front edge to the subframe. (you could have both suare and round stlye inner guards to suit the outer guards you have fitted that day
)
Now you would be able to lift the bonnet as per normal (or modify the hinges for quick release), then flip the front forwards (or remove it), and then remove the inner guards.
It all depends on how much stuffing around you are prepared to do.
If all that sounds too hard, have a look at this front end replacement I am doing.... (just a quick overview)
Click on pictures to enlarge them
Here you have a body with the front end removed...
1. Take a Rover Mini front cut...
2. Cut away all the bulkhead, leaving the inner guards, outer guards and front panel
3. Leave an overlap here for permanent welding (or mounting brakets for easy removal if desired)
4. And you get a car llke this
