[Exim] question about spool files

how compatible are the spool files between 3.36 and 4.14? If I copy to
the newer spool, will I lose anything? (I realize I may not be able to go
the other way).

Is all the necessary state information (regarding who has been delivered
vs who hasn't, not retry info) stored in the spool files? I know smail
used to use its msglog files as a holder of state information. I want to
confirm my belief that exim does _not_ act that way.

Thanks
--John