How are achievements/rules triggered?

We just upgraded our site from 8.0 to 10.0 and are just using the OOTB achievements for now. Can someone confirm that the achievements will only trigger/apply to the member when a particular action occurs? For example a member already has 16 friends (from before the upgrade) but did not receive the achievement for having 5 friends. Is this because the achievement is checked when a friend is added? Meaning, if he makes another connection/friend in the new version, will he then get the 5+ friend achievement?

Thanks,

Jesse