Making irreversible changes

OK, nothing so dramatic as the title might make it sound.  I'm designing hair and makeup for a stage adaptation of Pride & Prejudice and several of the actors will be wearing wigs.  We ended up ordering a wig for one of them; right texture but wrong shape.  Merely pinning it won't work because there's too much volume in the back & sides; trying to pin it smaller would merely leave it looking matted.  The challenge is to take this:

and trim it to approximate something like this:

Wish me luck!