Can I actually keep some Gliffy diagrams or go back if I do not like the result? How safe is the conversion process?
The Gliffy Mass-Importer creates copies of all Gliffy-diagrams and converts them into draw.io diagrams. No diagram gets deleted and all Gliffy diagrams will still be available in the attachement of the confluence page.
Easiest possibility to change back would be to roll back the confluence page version as a new version with each saved edit of a draw.io diagram is created. That way you would get back all Gliffy Macros on the confluence page. A draw.io Gliffy importer is not available.