Post-update check - Preventing user from changing it's own role (how to access pre-update values in post-update check?)

[franciscolourenco]

Is it possible to prevent a user with a role from changing it's own role to a certain role?

This user should have permission to change other people's roles.
But we want to prevent the user from promoting themselves. Wouldn't we need to have access to the previous value of the role in the post-update check? Is this possible?