The forum you posted too is just fine.
There's a number of issues at play here... I'm not an attorney so I'll leave legal considerations around merging, entities and rights to someone else [or really for you to take offline as those are hardly going to be covered in a post].
What strikes me as your big issue here is intent. That you own both sites is irrelevant for purposes of what a user has signed up for... yesterday I belonged to site A where I intended to participate and interact. tomorrow I will probably not be ok with a bunch of messages from site B.
If the two sites are very close in nature this will go over better, if the two sites work similarly it may even be a smooth merge but if not it will be near impossible to consider the old database opted into the new one. Even if you wrote terms that gave you the right to make the shift the blowback will be terrible and businesses grow from happy customers, not legally created ones.
When you have a disparate merge the right thing to do is invite the base. Second to that would be a time based move so recent members shift, old ones get an alert. Forwarding the urls with a transitional / explanation page can help pick up more. No matter how you slice it be honest and make the change in the open so people can decide what they want.