IIRC there's 3 transformers in an 11 coach 390 and 2 in a 9 coach and in theory 1 in a 5 coach but 2 in a 6 coach. As such that rules out shuffling around coaches or shortening some units to 6 to run in pairs to allow others to run as 11 coach units.
You could create 2 x 11 coach units by using the transformer coaches and two other coaches from a 9 coach unit. Leaving the two driving vehicles and 3 other coaches.
You could then build a new fleet of 7*11 coach of new tilting trains but such an order would be small and therefore expensive.
Such a change would likely only be required of HS2 isn't built at all.
Where HS2 is built the more likely outcome is scraping 3 of the coaches from the 9 coach units to create a fleet of 21*6 coach units which could be used to run services which split/joined to serve more places, but only required 1 path out of London on the WCML, or to better match loading capacity.