The only thing that comes to mind is SMB versions? .. i.e. old NAS was configured for SMBv1 which requires the Kodi SMB client on the RPi to be configured for min/max SMBv1 (else it connects at SMBv2 and cannot downgrade the connection to SMBv1) and new NAS has defaulted to SMBv2/3 thus prevening the Kodi SMB client trying to use min/max SMBv1 from being able to connect.
If that's not the right blind guess, as a minimum you'll need to divulge what protocol is used by Kodi.