Using alternative BOM with PR

hello

I'm trying to use the option of alternative BOM in purchase req.

I have a subcontractor which i'm sending him sometimes material X and sometimes material Y, depends what i have in stock, and get from him the same finished product.

The diference between X and Y is the package which i'm sending it with.

The alternative BOM defaultly brings to PR the first BOM (with material X) from transaction SC03.

I'm trying to change the BOM in the PR to the alternative BOM with material Y, but the field is unchangeable.

Is there a way to change between 2 alternative BOM's in the PR or in the PO??

Thanks

Itay Arad