The old parsercache hosts are running Jessie, but the new ones (T195878 and T195876) shall run Stretch. We don't know yet if we have to change anything in order to achieve this, but we know that we should test it out.
As codfw is a secondary datacenter now, and pc2006 is not in use (except replicating from pc1006) it is safe to reimage the entire host with stretch and if any problem emerges we could fix it before the new hosts are getting installed.