The ADMX templates are used only by the Group Policy Editor to show you the settings in a friendly way.
When you close the Group Policy editor, the options that you chose in the GUI are stored as registry values (or various format depending on the plugin).
So, nothing bad will happen when the policy syncs (data are in a "standalone" format, admx templates are not required by the clients to determine which settings should be applied).
However, you won't be able to see/edit the settings in the GUI if you open the policy editor on the DC with the older templates, additionally, this warning will show up in the GPO HTML report if generated by this DC:

I strongly recommend creating a Central Store for GPO Administrative Templates, with the central store you don't need to updates ADMX templates on each DC/computers with the Group Policy management console
The Central Store is a file location that is checked by the Group Policy tools by default. The Group Policy tools use all .admx files that are in the Central Store. The files that are in the Central Store are replicated to all domain controllers in the domain.