One thing I’m not sure of is what happens if I want to switch registrars in that time—will the full 10 years of ownership transfer to the new registrar?
If you switch registrars, your domain validity continues as before. Your registration information is with the registry for your TLD, not with the registrar alone. So your 10 years of (future) ownership will carry over. Two caveats to note. The first is that you can’t transfer a domain within 60 days of purchase or renewal. The second that I’ve seen is that a transfer to another registrar requires a one year renewal for the domain. So I’m guessing you may not be able to transfer a domain that’s already at 10 years (even taking into account 60 days after the renewal to keep it at 10 years). But if your domain is at nine years, you’d surely be able to transfer, and it would become 10 years at the time of transfer.
The expiry date never changes through all these actions.