Merging forums is always a little risky, especially if your forum as been established for a while, and you have the same regular people chatting on there. Merging with another can cause a few problems, as a forum is a community, and when you all of a sudden introduce new people in there, some might not take it so well.
What if your 2 forums have two different crowds that may not get along together? Another thing, many people are afraid of changes and there are people who have tendencies to quit if a change is going to remove them out of their comfort zone. It is risky but the final decision lies in your hands. You have to remember that this is going to alienate a lot of people. You are going to experience a damage control transition before your forums will recover. I experienced the aftermath of two merging communities before and the result is far from desirable. You have to prepare your members so they can gradually adapt to the change.
I think a lot also depends on why your merging your forum with another also. If it's for the benefit of the forum and your merging with another so it doesn't close then will be accepted more than if your just doing it for your own benefit.