It's a big win, but we stared from a big loss. We have about 20k downloads a month of our free open source software, so the amount of growth potential is huge.
What I think worked:
The marketing emails - let current members know the forums are moving, notifying them of the upcoming freeze for migration, announcing the new community is live, and then a follow up IF they didn't log in to the new space.
We reinstated our monthly live events - a Tech Talk Live and Office Hour session. These were done sporadically in the past - we've revived the regularly. We used an Office Hours session to promote the community.
We migrated our internal developer blogs (we had 10 or so employees with their own blogs on our site) into one blog with several writers. They earn points in the space for blogging and they can actually see their stats on readership, etc. So members don't have to check several blogs for info - they can just follow the blog relevant to their product.
Honestly - it's been mostly consolidation and replacing broken functionality.
Growth this year will be an entirely different ballgame!
I will say something that I haven't quite figured out. I'll actually post a new topic for discussion!