How to work with SharePoint Designer list forms that have more then 5000 items

Hi,

We created some forms (SharePoint Online list)  in the past using SharePoint designer four years ago, now there is a need to update those forms. We are not able to open the list forms and getting this error  in the designer - "The attempted operation is prohibited because it exceeds the list view threshold enforced by admin".

If we copy the list without content to another dev site (that we are), Can we copy it back with the list forms in Prod without affecting items in the list?

Please advise. 

Thanks,

-Sal