It is possible to compare 2 quality profiles to understand the differences in their rule sets:
The comparison service shows all rule differences (rules in one profile but not in the other, and rules with different severities or with parameters differences):
Comparing Versions of a Profile
Every rule change made to a quality profile will appear in the profile change log:
You can use the changelog comparison service to see the differences between any two versions of a single profile.
A new version number is stamped on a profile at the next analysis after rule changes are made. Each project analyzed with that changed profile (not just the first one) will be stamped with a Profile change event.