Hi,
Welcome to ask here!
There will no impact on the existing GPOs you deployed before.
The ADMX files are used only when you are when you are editing GPOs in GP Editor. When you expand the Administrative Templates node, and you see all of those folders, sub-folders and policy items–those correspond to GP Editor reading each ADMX file it finds, and building that tree of nodes dynamically.
Once you are finished editing that GPO, those ADMX files are not in use at all.
When client computers or users process Group Policy, the ADMX files are not used at all.
They are not stored in the GPO, they are not downloaded by the client that is processing that GPO, and they are never referenced during GP processing.
ADMX files central store is only for the easy management.
Best Regards,