How to detect an upgrade from an older version of a package

[Thomas Goirand <thomas@goirand.fr>]

Hi,

I want to make an update to libapache-mod-log-sql that needs some
maintenances in it's SQL databases if we need to upgrade. Namely, it
needs some table structure upgrades.

I know many "hacks" to know that I'm upgrading (like doing dpkg -l,
etc.), but what is the correct/policy way to know from what version my
package is upgrading, so my postinst can run smoothly?

Thomas Goirand