[p4] Moving a changelist across to a new branch?

[Troughton, Robert]

Imagine a situation where a programmer has been working on a change
that's turned out to be much bigger than he'd first expected... his
changelist expands to, say, 100 changes, a mixture of adds, deletes and
edits, and it looks like the changelist will get bigger still before
he's finally able to check it in.

So... after two weeks of work, he decides that he should've created a
branch for his changed.

Basically... how easy would it be to create that branch from the current
"head" version on Perforce and to pull his changelist across to that
branch instead?

Sorry if I've explained this badly ... I hope the question makes some
sense?

Robert