Bracket our guild is in only changed once after 4th season, since then including this season it stayed same.
But heard from others for them it changed already before 5th and also later or more as once.
Someone told me there is a logic that is based on guild activities. Activity points from all guilds in 1 server are summed up, then next server is added, etc and as soon as a certain value is reached, no new server is added to this bracket. Then it starts with next guild, etc.
No clue if that's true or not...but at least it's not like if 1 bracket is changed all others are changed as well (but obvious the "neighbours" :at least 1 bracket before and after