Strange reconstruct problem with version migration

[Tim Champ]

Hello all.

We've setup a new cyrus backend server with 2.3.15. (as a part of our 
cyrus mail setup here)  Our current version is a patched up 2.3.8.  When 
we xfer users from an "old" backend server to another "old" server and 
then reconstruct, we have no problems.  It works as we'd expect - purges 
expunged mails, etc.

When we do this from an "old" backend server to the "new" one, it always 
sees the expunge file as not valid.  The message:

"Unable to verify header - deleting: $MAILBOX_PATH/cyrus.expunge"

The access to the mailbox, etc, all seems to work fine, except that any 
mail that was previously deleted and not removed via cyr_expire (we do 
delayed delete) returns to the mailbox.  This concerns us as we're not 
sure why it wouldn't understand the expunge files.

Although we could not reconstruct post-move (we've done it for cleanup 
purposes historically), it worries us that we'd place this version of 
cyrus on an existing mail server and have this issue the first time a 
reconstruct was needed.  (undead mail, as we've termed it)

Any ideas?  Still doing testing and trying things, but at a loss right 
now.  If more information is needed, I'm happy to provide it.  We've 
googled around, and searched the mail archives, read documenation, but 
to no avail.

Thanks!

Tim