Just one opinion, would base the move based upon the coach's recommendation, gym's "philosophy", and your daughter's goal. If it is important to your DD to have a phenomenal season versus a challenging and maybe lower scoring one (and the move from 8 to 9 is challenging even for the accomplished level 8 gymnast at least from a lay perspective) then yes repeat 8. Is being with her teammates in a certain level and having a strong team that wins alot important? If most of the level 8 team is repeating and she doesn't, is that OK for her or vice versa? Will she train level 9 skills throughout the season if she repeats or be locked into routines during the 4-5 months or so of competition season and maybe get bored? Level 8 does restrict more difficulty.
My DD is nearing the end of competitive gymnastics. Her coach said she could repeat 8 again and do really really well (she didn't qualify for regionals level 8 ) or go the tougher route and recognize she wouldn't score well but make it to level 9. She chose the latter path and it has been very tough (because her level 8 season was not that strong) but she worked 110%, mastered some difficult skills, managed to get some decent scores, qualified to state, achieved the USAG level 10 mobility score even though she will not being going to level 10.