Have to apologize, but have as yet not delved further into this. Some factuals:

1) The client is attached to BAM
2) All project are being attached via BAM (WCG special, first time it can't)
3) On the project list page the field is ticked to "attach new host by default?" on the project list page to all new clients.
4) The names of the devices on the Hosts list is Unique or has duplicates? (Mine shows one host with a Windows and a Linux ID, rest are unique)
5) During client update, the data_dir info is retained i.e. same projects have remained attached, without detaches being logged in message log or on results.

A) BOINCStats knows the CPID, which you can refresh in the account page
B) BOINCStats knows the numeric account ID of your projects.
C) The BAM AMS functions box is selected. (My contact frequency is set there to ones per 24 hours)

If those settings are equal and behavior is same, then no idea why this hits some, but certainly not me.

I've noticed one thing: The hosts CPID, not the account CPID on the windows/linux dual boot is different, logically. I wonder what happens if this ID of the old ID is copied into the files on the host [after stopping the host].

The other thing I do not grasp is the BAM host ID. Mine are all 6 digit even the one created on August 5.2010, so why these 7 digit host IDs... found there are 2 and can correllate the 6 with the 7 digit number, both showing with same host CPID, the 7 digit number probably the 'public' number.

Well, then I fear it's really at the project level not recognizing it was a pre-existing host and then BAM just reporting what it's being fed from the projects. Given that all projects except WCG have the device merge feature I'd expect for the stats afterwards normalize over time.

Basically, if a client is installed and it does not find the old data_dir and account_xxxx.xml files and the client then talks to the projects servers, they think it's a new device. That's why I go off-line, make a data_dir backup, so I can correct if things go wrong and only then run the upgrade, then verify all is still same and then go on-line again.

PS, switching from 32 bit to 64 bit changes the client install area and is known to cause issues.