in April my developer sub will expire. For economical problems I need to wait some months before renew it and I know I can not update the different components and modules like CB Subscription since I'll renew it again.
No worries, we understand.
My question is: meanwhile, can I, at least, update the CB core component, using the free version but continue to use the old component and modules already installed on my website?
Yes.
I mean, the CB Core free version, comunicate with CB Subscription and others without update them but only the CB component?
Yup, that'll work fine. We provide backwards compatibility within our APIs throughout the lifetime of a major CB release. So you shouldn't expect issues regarding the core being incompatibility with a plugin until CB 3.x, which won't be for quite awhile since we'll be doing a full rewrite with CB 3.x.