A new paper has been published in the Journal of Information Processing. This is a full extended version of the short abstract from JCDCG3 last year.
Title: Hardness of Reconfiguring Robot Swarms with Uniform External Control in Limited Directions
Authors: David Caballero, Angel A. Cantu, Timothy Gomez, Austin Luchsinger, Robert Schweller, and Tim Wylie
There is also an Arxiv version: https://arxiv.org/abs/2003.13097
Algorithmic Self-Assembly Research Group