I am looking for a 2nd hand set of Beolab5s and recently saw an add for a used set of Beolab5s. It stated that the speakers were 3,5 years old but had been "upgraded to 2009 version" at the factory. What would such upgrade(s) include?

I believe the 2009 version got the following modification:

The "new" beolab 5 have a new volume chip and the software version is 3.00.

From Keith Sanders in another thread. I'd think that the speakers have then had the PCB replaced?

"The latest release is Software 3.00a and provides the following improvement is only for production due to update of the volume control IC and new design of the according PCB meaning your units cannot be updated to version 3.00a unless the associated PCB is also replaced."

Somewhere recently I read that the crossover points were updated. Maybe this came with the SW update.

I was almost certain I heard a (fairly small, but not insignificant) difference when the store in Bath got in a newer demo pair!

I've gotten the following reply from B&O (in Danish):

"Det er korrekt, at der var en softwareopdatering i 2009 til version 3.00. Denne opdatering har ingen indflydelse på kvalitet eller lignende, men var udelukkende relevant for produktionen."

Essentially, the software update did NOT have any impact on sound quality, it was just a production issue.