merge boards into a trustedlink

Hi, I have to decide if is better to change this situation and merge 3 boards, I need your help
I have:

• 1 trusted site, online since 2001, listed in dmoz, with 2 forums in 2 subdomains:subdomain A, with vbseo installed, with specific target, many specific threads; subdomain B, without vbseo, generic forum. many threads
• 1 site concerning travel, not listed in dmoz, vbseo installed from two months, but without many threads

In order to increase travel section, I think to merge 3 boards and make:
subdomainA --> mainsite.ext/forumA
subdomainB --> mainsite.ext/forumB
travel site --> mainsite.ext/forumtravel

Advantages: more users that could post in 3 forums without new registration;
But...
- big work for merge boards
- in trusted site I cannot use adsense (I use yahoo!ads), it's more difficult to show different banners for different sections

What do you think? What to consider? Thanks!

if they are different topics, i wouldn't even consider this an option.

the bad part is that there;s no easy way to go back if it turns out it doesn't work...

so, you mean that could be better differents sites for differents arguments?
For keys that I'm looking for, I see some better place in serp with trustedsite.ext/travel than www.travelsite.ext
Othre main question: if merging user will spread possibility to growth the community with more threads. The main community grows, the travel community is (too much) stable

did it. Old site disappeared from serp immediately, without any 301 redirects. New board into trasted site is taking more visits