All players born January 2001 or after don’t need to be registered for PL so they’re fine on that end as majority of their signings fall into that category. The Cat B players for CL (Broja, Hall etc) can be interchanged because they have been at Chelsea for a certain amount of years and don’t need to be registered. Madueke, Mudryk and even Fernandez don’t fall under this category due not not being home grown so would need to be registered as a senior member and take a spot from Ziyech or whatever. Even if Chelsea sold 10 players who were registered, they can only register 3 new players for the knockout stages.