If you have several of you in one booking. Restricted booking class (I class). Later award availability (Z) opens up for some but not all the passengers. Can you MFU just some of them?

I assume this means splitting the original booking into 2, and MFUing one booking and leaving the other as it is? Will BA do this, if you are all in a non-changeable (except for a fee) booking class? Or will they try and charge the change fee? Or will they refuse on the basis that there are insufficient reward seats for all passengers?