I would start by getting the data off those devices. If those devices are remote, this is the easiest way I've found.
- Install Files from the Play Store. https://play.google.com/store/apps/details?id=com.marc.files is the direct link.
- Switch from icon view to list view for easier navigation.
- Tap on "Android", then "data", then "org.odk.collect.android".
- Long press on "files" folder, tap on ⋮, then tap Compress icon.
- Long press on "files.zip" file, tap on ⋮, then tap Share icon.
- Chose Gmail, it will create a new email with files.zip. Send the email to yourself.
- When you are done, long press "files.zip" (not files!), tap on Trash icon. Click OK to confirm.
Once you have all the odk folders, then you can use ODK Briefcase to push them to Central.