You have several choices as I see it.
1. Just update the old domain and put ads on it.
If you are worried about the time investment, go ahead and work with the old web site. Put adds on it, scrap the new idea or put it on the back burner.
2. Update the old and move forward with the new site. The two sites reference each other.
Work on both sites concurrently as partner sites. More work perhaps. But worth it in the long run. Does the old one have decent traffic? That could get the new one off it's feet.
3. Scrap the old and go with the new.
Propably a waste of a good domain so forget it.
4. Expand the old to include the new.
This is probably what I'd do. One site to manage with some page rank to work with. If they are similar enough, surely it can be done in some logical fashion. Put your ads up and go with it.