You need to import them in NCM 7.4 UI -- import from file.
If you have tons of custom device templates, you may consider to upgrade from 7.2.2 to 7.3.2 first, copy the device templates to the DeviceTypes folder of 7.3.2 and then upgrade to 7.4 -- in this case, device templates will be imported automatically. However, I'm not sure if that extra upgrade steps outweights the effort with manual import.
Jiri