I've had as many struggles with under-placement as with over-placement. We had frustration, boredom, lazyness, and overall poor attitude toward school work as more and more time went by with my dd placed too low. I know it may not be the same advice that others have given, but I would really bump her up, if it were me. I wish I had much sooner with my own dd!
I know it's not ideal to skip the remainder of Beyond, but I don't think you'll be gaining as much by sticking it out as you would by placing her up right now. If you can keep her spark for learning, keep her enjoying it, keep her skills growing, go for it! My dd missed out on the last 1/2 or more of Bigger, but we didn't notice a gap at all. Like you said, sometimes their skills grow just by getting older and moving out of a guide that way. I don't think it hurts to look at the placement chart with fresh eyes when your questioning whether your children are correctly placed. Pretend you're new and coming to HOD and think about where you would place her; and then take the plunge! Go half pace if you need to ease her in to the higher requirements, and I think it's okay to ease her in with the writing requirements, too. That is a big jump, even if she is ready for it!
Furthermore, the opportunity to combine would make this an even better option in my eyes. I've looked and looked for a way to make combining work for our family, but so far it hasn't presented itself.

Depending on where your older child is in Bigger, you could have her go half pace for a few weeks for younger sis to catch up, or you could possible just dive into the guide wherever big sister's at, if you're still in the first few Units.