Extended Warranty terms begin on page 29, with extensive explanation of how Citi's benefit applies when there are other warranties:
https://www.cardbenefits.citi.com/~/...R20_FINAL.ashx
Also read the Additional Terms at the end of the document:
These benefits do not apply if Your card privileges
have been cancelled. However, benefits will still apply for any
benefit You were eligible for prior to the date that Your account is
suspended or cancelled, subject to the terms and conditions
of coverage.
I read this to mean that if you close your account the coverage ends, but pending claims will still be processed. Downgrading to another card type is not explicitly discussed.