Referring to the new Achievements as "upgrading" is likely to be inaccurate. From what we have heard so far, Fandom is trying to integrate it into Gamepedia's achievement system. As such, I expect that it will work noticeably different once it is "converted".
I also want Chat to come back. However, I think Fandom is more likely to drop the feature than keep it.
It is a custom feature so they would have to convert and maintain the code; something they are trying to avoid with UCP.
They really haven't been developing it any further beyond its initial state. This is evidenced by all the additional scripts on the dev wiki.
A good portion of wikis ditch Chat for Discord anyways.
Fandom itself prefers Discord as evidenced by the removal of Chat from CC and the holding of the AMAs via Discord.