Hey Emily!
At this stage, you would need to duplicate and move those subitems in order for the original subitems to stay fixed to the original group. I understand that the ‘copy’ text isn’t ideal and encourage you to add your vote to the existing feature request to remove this here 
I have also come across an older thread where @JCorrell was able to solve this using Integromat - Item Copy function - removing the added " (copy)" - #5 by jenstratton. I recognise using a third party tool isn’t the most ideal way forward, but we hope this can help in the meantime.