[CentOS] Old and new package version numbers during RPM update

Hi CentOS folk,

In an RPM post-install script, is it possible to know the previous
version number, and the new version number of a package if it's an update?

I need to know this, because for a certain package, if updating from
version 1.x to 2.x, I need to run a program to convert the config file
of the package from version 1.x format to version 2.x format.

I've looked at SPEC file documentation, but haven't found anything relevant.

Regards,

Anand