Thanks for the assistance here. There are a couple of hundredth invoices. It would of been too much of a manual effort. I found a faster way to do it with a workaround.
1. Created a saved search to identify the specific Invoices
2. Created a "scheduled" workflow - used the SS to call the record and set the custom form. It worked 🎉
I've set the workflow to testing mode so I'm the only person who can execute it. It can be re-used in future if needed