You must earn merit to change your rank.

The Bitcoin Forum has been going through various changes, expansions, plans over time.  From 22 November 2009 to 17 June 2013 the forum rank fluctuated with no activity.
At that time, the rank would change only when the full member had 100 posts, the senior member had 250 posts, the hero member had 500 posts and the legendary member had 1000 posts.

18 June 2013 to 23 January 2018 requires activity with Bitcoin forum posts.  Brand New requires 0 posts 0 activities, Newbie 1-28 posts 28 activities, Junior members 30 activities, Members 60 activities, Full members 120 activities, Senior members 240 activities, Hero members 480 activities, Legendary members 960 activities.

Since 24th January 2018, activity as well as merit system has been introduced.  Now it's not just about activity but it has to have merit number.

Brand new/Newbie (0 activity, 0 merits)
Junior member (30 activity, 1 merit required)
Member ( 60 activity, 10 merit required)
Full member ( 120 activity, 100 merit required)
Senior member (240 activity, 250 merit required)
Hero member (480 activity, 500 merit required)
Legendary member ( 960-1000 activity, 1000 merits required)

Please visit here for more information: https://bitcointalk.org/index.php?topic=5170032.msg51993801#msg51993801