[Dagoor]

i did it this way:

first you need to create that usergroup where you want the users with specific number of posts to be transfer to.

then in AdminCP>User Groups>Promotion>

click on Add New Promotion

for the first option "Usergroup" select your user's default group (registered?)
and then in front of "Posts" set the value you want

for the "Promotion Strategy" set it to "posts"

let the Promotion Type be primary and finally "Move User to Usergroup" select that user group that you just created.

since you already have members... i think you also need to REBUILD USERS in Maintenance>update counter im not sure. i dont remember how it did it