I have a fairly large Magnolia that is at least 25yrs old. It is in a
position where it will have to go because it will interfere with planned
building works. It has also become quite leggy due to neglect and I know
that they cannot be hard pruned.

However it has put down a couple of branches that have taken and
sprouted new trunks which are now about 6' high and 1" or so diameter.
If I cut these off from the main plant are they likely to survive in a
large pot until they can be planted elsewhere after all the work is
completed?

Andrew