I think how it will work is that for levels 80-90 we'll get a new party, separate from the 70-80 trusts... I think for example ryne won't be able to join because for this level bracket she's been replaced by estinien in the party. If it doesn't work like this, leveling all characters is going to take super long and be quite a mess... imagine a new player that just got to endwalker's endgame. They'd have to level y'shtola all the way from level 71 if they want to do a lv 90 dungeon with trusts... I hope that's not how it works and the new party starts at level 80.