As others have said, Pendolinos can't simply be chopped and changed. You can add additional carriages, but you can't remove carriages without making suitable modifications to these. It's probably quicker and easier to do another order for to make up the remaining 20-odd 9-coachers to full 11-coachers. Perhaps a few extra to replace the under-the-wires Voyager operations so that the only Voyager operations are for North Wales and Shrewsbury. Then the Voyagers freed up could go to CrossCountry, who are really desperate for more, longer trains.
I realise for Pendos to be re-jigged with existing carriages major mods would be needed at Alstom. But this still may make sense.
Don't forget that the class 390 is now outdated as regards building any new sets. It doesn't pass current regs for new build trains so new ones cannot be bashed out anymore.
Whether new build 390 coaches can be added to existing 9 car sets I'm not sure. May not pass the regs either. However I suspect that braking down some 9 car sets and (painstakingly?) updating systems and adding them into the remaining 9 car sets to make them 11 may well pass the safety regs as they are not new build coaches/cars.
For Alstom to produce any new Pendos would need a new design of train class based on the old and would need to be worth the trouble in terms of the number of sets ordered. Whereas taking the trouble to break down some 9 car sets and making up the other 9 car sets to 11, although would need probably weeks of work per 2 cars added, may be more economically viable.
If we are talking ordering new trains, it only make sense designing a new fleet if the order volume is large enough. So naturally we would talking about replacing the Voyagers. New diesel only tilting trains couldn't be justified environmentally so bi mode would be the only sensible option.
Hitachi seem to be the favoured bi mode suppliers at present and have facilities in the UK and abroad. If they were able to design a tilting version of their current hi speed bi mode type train it could be the sort of viable solution that is needed.
The whole West Coast Voyager fleet could be replaced with these Bi mode tilting trains + a few token extra sets to make up for the Pendo lost at Grayrigg and a few others for any new workings.