If she is able to swing either way and is TRULY prepared to compete Level 6 reasonably successful, meaning has all the skills and can score competitively (not meaning win, but get scores she is pleased with), then I feel the decision will lay on her character.
Is she one that needs tangible success to feel good about herself (medals, good scores, etc)? If so, hold back while working Level 6/7 preparedness.
Is she is one that can set goals personal to herself and honestly feel good and energized by succeeding in these goals, i.e. who cares about scores and how I place, I got my "whatever skill" in practice! Then she could get through a ho-hum Level 6 season okay. She needs to be rather balanced in what drives her. She needs to care and be successful enough to score out of Level 6, but not be too worried if she isn't scoring over 33.'s (or whatever) and not placing. AND fundamentals for good optional gymnastics MUST BE encouraged.
It's going to take some time to get through either repeating as a successful Level 5 or moving through to a so-so Level 6. If she defines fun and success in scores and placing then keep her Level 5. If she defines fun as hard work and mastering new skills, and she is ready, then move her Level 6.
FUN should be first, or she will lose interest.
That was a lot of talking in circles, but I hope it helps.