As said in the other ticket, there are extensions for schemas but not for forms, so you must modify the factory form for a schema extended as for factory schema.
Moreover, during a postupgrade operation (upgrading towards a new build), the postupgrade log file(s) (one per instance) warn(s) about the discrepancies between the current version and new version of all objects (schemas, forms).
Actually it keeps the current version and the admin people have to choose what action to do in the Admin>Settings>Conflict management node.
You can keep the current version, accept the new one, or merge manually in order to keep what you customized and adding the new elements brought by the new version/build. Did you accept the new version of schemas/forms directly or did you make a backup (especially doing a export package (xml file) or by copying the code into text files as backup?
In case of no backup of customized forms losts, good luck to redo your work.