Thanks orbitmic, definitely the change would be after outbound travel, and yes, a high degree of flexibility and quite a lot of notice can be given.

I wanted to check that a TA can in fact split a PNR at all, and that the same thing would apply. I'd have assumed that as they have embedded the voucher on the original flights, and booked into a revenue bucket, the person travelling back on those flights would be fine, but the person switching probably wouldn't.