Is it possible to force just one Form into Classic Mode?
From the docs:
Use auto-detect to automatically return to classic view
As an admin, you can choose to show classic view only, or use the new experience but automatically return to classic view when certain features are used. These features are:
Metadata navigation and filtering – All libraries on sites that have this feature enabled are shown in classic view.
External data columns
Publishing columns (Publishing HTML, Publishing Image, Publishing Hyperlink)
Limited-access user permission lockdown mode
JSLink code on fields
CustomActions that include ScriptBlock or ScriptSrc properties
Ithink it is not possible