Photos and icloud sync

Hi,

I have an old iMac (late 2013, with Catalina OSX) where I uploaded some old pictures to Photos before I was syncing with iCloud.

In the result I have now all of the devices (iphone, Ipad ect) syncing all the current photos to iCloud but only iMac shows a few years of old photos that were not synced before.

The 'iCloud synced photos' show on the iMac but older pictures do not show on any other devices.

Now I need to get rid of the iMac and I'm not sure how to add those 'old' photos to iCloud sync.

Should the old photos add to the sync or I need to manually transfer them to one of the 'current' devices?

Cheers

Mac