I've just updated my firmware from the iPad ( iOS 26 developer beta 3, Eve App 6.3.9 ) and it worked, though: this is the first time I've updated firmware in general, so it migrated the Thermo to Matter - not entirely sure if that's comparable to your problem....

Which firmware is your Home app showing? At the moment it is apparently version 3.5.1 and version 3.5.2 is supposedly in the queue. According to Eve Support, they are still waiting for approval from an authority in the USA. A Matter update via the Eve app is probably not possible, it can only be done via the Home app (if the update is offered).