I am transitioning both my professional and personal GPG keys. This transition document (in full, below) and both updated keys are signed with both old and new keys for both personal and professional accounts to validate the transition.
If this is all greek to you, GPG (or PGP) is a way to encrypt your email so that only other specific people (who must also be using GPG) are able to read it. While we think of email like regular mail, with a level of privacy like something in an envelope, the reality is that it's better to compare it to a postcard. If you're interested in getting started, I highly recommend EFF's excellent PGP guide, and Mailvelope is a super-easy browser plugin to help get you started in more secure webmail (it works great, for example, with gmail).