From Reddit: NO level sync when entering a dungeon with less that required players!

[Renik]

You would not get roulette reward, because premade parties can't start low level roullete (or how lv17-59 levelup dungeon roulette will be called again). What remains is the exp from the enemies. And look how much exp from a lv 17 enemy you get with lv 50 ^^

It will be more for farming gear for glamour or GC seals. Everyone who want exp will still do roulette.

Last i read about it, current lvl50 and 51-59 dungeons will be part of the high lvl roulette, but the option to enter uncapped in smaller groups will be in the DF, so i guess activating it will prevent the use of roulette, or at least this seems to be the more logical way to manage it.

[Felis]

Last i read about it, current lvl50 and 51-59 dungeons will be part of the high lvl roulette,

Yes, but you would need to be overleveled and/or overgeared to do a dungeon solo with success.
Also the current lv 50 dungeons get an own roulette, seperated from the lv 51-59 dungeons.

[ShoeVsYou]

Last i read about it, current lvl50 and 51-59 dungeons will be part of the high lvl roulette

This is incorrect. They have specifically states that lvl50 dungeons will remain in High Level Roulette, while 51-59 dungeons will be added to the Low Level (or leveling) Roulette.