How to extend AppleCare+ early?

My AppleCare+ agreement on my MacBook Pro expires in mid-July. At present, checking my coverage shows everything is fine as expected. However, a search of my options does not reveal any means of extending my coverage early, before the expiry date. Do I have to wait until the expiry date, and within the renewal window (45 days, if I recall correctly) before the options change to allow an extension? I'm in the UK, for clarity of the relevant rules. In the past, I've traded in the old machine for new, plus new coverage.