I knew that question was coming! I was going to write a quick guide, but had to walk the dog before he destroyed the house!
1) If you did not use the "I have 10+ members, create a custom forum" option for your group discussion, your choice is simple. There's a toggle in the group settings (Edit Group) to switch discussion on or off. Today's upgrade has turned them off by default (annoyingly!)
2) If you DID use the "I have 10+ members, create a custom forum", then your custom forum still exists, although the new group format doesn't link to it. You can
---(a) Keep the forum, and provide a link in your group description/header. If you do that you're best advised to turn group discussion to OFF, otherwise you'll have both types, which could be confusing.
---(b) Simply use option 1 above, and ignore the custom forum.
The new default discussion (option 1) allows for threads etc., now, so the only advantage to keeping a previously created custom forum is if there was a lot of conversation you wanted to keep. Or you could link to it as an archive and activate the default group discussion system.
If anyone didn't bookmark their custom forum, and need the link, let me know (remember to tell me what your group name is). There were only 13 people who used that option, and most of them had almost no conversation in them, so it shouldn't be a major issue.
As to why the change? The previous option was a hack that I installed to provide asked-for functionality. The latest vBulletin upgrade provides similar functionality by default, and better coded, too.