Hmmm, sorry to do this, replying to my own post.
It turns out that in OMV4 I had unionFS plugin installed. I uninstalled the plugin prior to the upgrade, but forgot to remove the shares and file systems.
I deleted these in Services>SMB/CIFS, Storage>File Systems, and also Access Rights Management>Shared Folders (I think), and now I can apply the changes successfully in the GUI.
Leaving the previous post there in monument to my own carelessness.
I still have the following errors peppering the system logs which I need to investigate;
smbd[798]: process_usershare_file: stat of /var/lib/samba/usershares/ufsmedia failed. No such file or directory
ufsmedia is the name of one of my old unionFS shares.
EDIT: solved by the solution in this thread - process_usershare_file: stat of /var/lib/samba/usershares failed although I have no idea why.
Thanks for looking!