In my understanding some 50x (not sure which) are going to be retained for peak Southport line 6-car work until the platform/signalling work is done to allow for 8-car working at Southport. I suspect on the other lines, if affected*, which are generally a bit quieter, they'll be able to get away with not running double sets at all, as a 4-car 777 has quite a bit more capacity (albeit mostly standing) than a 50x.
* I seem to recall the starting signal at Ormskirk is very close to the end of a 6-car so it probably needs work too, unless it's possible to solve it by moving the stopping position nearer the buffers.