The oldest seat designs, by cabin:

ECONOMY
A319/A320 non-NEXT fleet. These Recaro seats first appeared in 2013. Installations began after the UA/CO merger announcement, but the densification & slimline project was commissioned by legacy-UA prior to merger. The grey seat covers are the oldest, found on the first batch of installs. Later installs are the more ubiquitous navy blue. These seats are not being replaced as part of NEXT. They are being reupholstered and the seat backs are being modified to add screens.

DOMESTIC FIRST
737-800 non-NEXT fleet. These Koito seats are original to the legacy-CO fleet, going as far back as the late 1990s. They are being removed and replaced as part of the NEXT project with a new model of seat that has dividers, seat-back screens and cordless phone chargers.

POLARIS
757-200 fleet. These BAe "Diamond" seats were a legacy-CO install, in the 2000s when CO decided to redeploy this fleet from domestic operations to international. The original 24 standard domestic first recliner seats were removed and replaced by 16 lie-flat "BusinessFirst" seats in a 2x2 arrangement. They have been refreshed and reupholstered in the past five years. No further modifications are planned, with this fleet scheduled to be fully retired in the next five years.