Protecting My Key

Here is the setup and my Theory:

I have been using PGP for years and I have a 2048 Public Key that the people I correspond with need to open existing emails from me.

I want to create a 4096 Public Key and NOT put it on the Public Servers.  Instead, I want to manually distribute via USB key to the people who will need it, but, I need to make sure that they can set this key to be my default, but, that they can still open existing emails with my current (soon to be old) key.

Is this possible?

Will PGP Desktop 10.3.1 allow this?

Will my 4096 key from a USB key to the local keyring even work on older versions of PGP?

I have a lot of "I don't knows", does anyone have answers?

Chicago