If you could upgrade now or soon to 6.0.2 that would be the best overall option.  Downgrading firmware will require a reset and restart of that member.  Best option is remove it from the group, open a support case and they can help you downgrade it.  6.x firmware sets disallow downgrades by default. 
I just received a new PS6100XV running FW 6.0.1 and have added it to my group with two PS5000E's and PS6000E running FW 5.2.5.  I think this was the first time I got a new unit out of five we've purchased that was actually running almost the latest and greatest firmware so I hadn't even thought about it before I added it.  I noticed that the settings on the firmware upgrade tab of my 6100 says "group does not allow firmware downgrade".  Does that mean that I could allow it to downgrade if I could change a setting, or that I technically can't downgrade from 6.0.1 to 5.2.5?  If I can't I'll just have to bite the bullet and plan my upgrade to 6.0.2 (starting with the new guy).Thanks for the clarification.  
Downgrade PS6100XV from 6.0.1 to 5.2.5