If you suspend your principal activity, you will keep the status of self-employed person for your secondary activity if you 

• receive an wage-replacing income from a social security fund that is at least equal to the amount of a minimum pension for a single self-employed person 
• protect your rights to retirement or disablement pension

The self-employed secondary activity of a person who suspends his principal activity does not automatically become a self-employed principal activity.

Pensioners
After your retirement, it is technically of little use to speak of a secondary activity. At that moment, you are part of the special contribution category of pensioners.