A bump, but maybe a clue for people converting.
I lost over 9000 attachments due to this, only the last 35 or so attachments were present. And actually I think it is a bug in the upgrade script. I upgraded from 3.8.9 to 4.2.3.
Some time later all my attachments were gone.
So, I did a new upgrade andpayed attention to the two known tables.
What I noticed that for all attachments but the last couple, the fields for height width and refcount had no value or "0" actually.
So, it seems to me that not all records are updated correctly in the upgrade process.
EDIT: Reason for this opinion: For the records missing the Refcount data, also the height and width fields contained no data, wehre the ones that did have a Refcount, also had a value in this field.
For who is planning an upgrade:
For this problem you can just go ahead. But disable the "Hourly Cleanup 2" first thing after the upgrade, and do not run update counters. Set the refcount to 1 as described here:
https://vborg.vbsupport.ru/showthread.php?t=320246
Then you can enable the disabled task again. And do all other things you have to do after the upgrade.