I am working on this for the request you posted and it is different from what you posted here. His site is missing attachments mostly before a certain time, not all. He has no attachment data in posts or anything else and I do not believe his issue occurred because of an upgrade.
As for your issue I have no idea how a simple upgrade would remove your attachment data and I have already done this 4.2.3 update on about 13 or 14 sites without any attachment issues. With that said I do not believe it to be global.
Are your attachments showing in the filedata table or post table? If not they are ever going to show in your posts even if they do exist in the attachments table.