[coffeefirst]

Because I bought it 10 years ago, when Google was a very different organization, and moving would appear to risk some email downtime.

[jamieweb]

You can transfer a domain with no downtime. The name servers will remain during the transfer, so as long as your DNS isn't dependent on the domain registrar, you'll be fine.

If you are using the registrar's built-in DNS hosting, move away from this first, which can also be done with no downtime.