The support policy on this has changed in the last few months. You can try, but They are now likely to tell the OP just to factory reset and start everything over from the beginning. From the official knowledgebase article:
SmartThings strongly discourages adding the Hub to another Z-Wave network. We cannot offer support for disconnected Z-Wave devices or the inability to add devices through the Hub as a result of including the Hub into another Z-Wave network.
This is a change from what they used to do, but unfortunately for some technical reasons SmartThings just doesn’t work very well either as a secondary or as the primary to another secondary. It meets the minimum requirements to be certified as a Z wave primary controller, but it doesn’t have some of the advanced features that systems which are designed for secondary controller management offer. Such as controller shift, which is what would change the SUCID.
I wish I had a better answer to give, but unfortunately there isn’t one. Once you’ve made it a secondary, you can’t get it back to primary without a full reset. 