Missing Attachments after upgrade
 

This is just a note for others who have had problems with missing attachments after upgrading from version 3.8 to 4.2.3.
It seems during the move of the attachments from vb_attachments table to vb_filedata in the upgrade steps in 4.0.0, a new column gets added called refcount but it is never populated.
Then, in 4.2.3 VB fixed a bug in a cleanup task that uses this column to determine whether an attachment is orphaned. Because none of the version 3 upgraded attachments had this column populated, the cleanup task removed ALL attachments.

Solution : Upgrade to any version less then 4.2.3 and update this column to make refcount = 1 using a SQL tool, then upgrade to 4.2.3.

Well people are having this issue after upgrading to 4.2.3 from any other version. Once the problem is already there it is not fixable as you mentioned so it is not a "solution". Ofcourse your instructions will stop the issue from happening in the first place possibly.