OneStream EmployeeChrisF1, If you are looking for ideas, thoughts, and tools to assist you with this change, my suggestion would be to pose this question to the community on one of the forums here.
In short, you should be able to insert a new summary dimension between the RootAccountDim and your detailed account dimension using the "Move Dimension" feature. You can then go through the exercise of removing the desired parent members from the detailed account dimension and creating them in the new summary dimension. Finally, re-establish the correct parent-child relationships to link the dimensions together.
This works because parent members (aside from Entities) do not have any data stored in the database. This is also the reason that base members with data cannot be moved between dimensions currently. There are different strategies and techniques if you do need to to "move" a base member with data up to the summary dimension for input there.