Upgrading BE 2010 R2

First Question:

If I update my current 2010 R2 with the latest greatest service packs and hot fixes.

I know that the remote agents need to be updated also but will they still communicate and function until i can get them updated?

Second Question:

If I upgrade my current 2010 R2 to 2010 R3.

Will the older agents still communicate and function until i can get them updated?

Third Question:

If I upgrade my current 2010 R2 to 2012.

Will the older agents still communicate and function until i can get them updated?

Forth Question:

How does the in-line upgrade from 2010 R2 to 2012 work?