Not sure if still the case: odd numbers for males and even numbers for females.

This also means a person’s Belgian RRN can change.

Yep, but they do not change on sex change, IIRC.

	▲	andruby 9 hours ago \| parent [-]
		I wrote a system for a medical lab decades ago and IIRC I did actually have to change some RRN's manually. It's been a while so I'm not 100% sure, but I do think the changes were due to switching gender.