You can move all of the content from Forum A to Forum B in the Control Panel:
v7.6.x: CP -> Content -> Manage Topics
v7.5.x: CP -> Content Management -> Move Topics

Then you can either delete the other forum or disable it so you can re-use it later.

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
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