Answers

In our market (Wisconsin) we normally call the electrician for that work. The problem is that few trades are fully integrated. Often times the vent size will change and the electrician will not normally do that. You will probalby have to have a HVAC contractor change the vent size. And then you might need to have a drywaller patch drywall as well.

If you want one stop shopping, you can have a professional Remodeler coordinate the whole thing and a skilled carpenter will change the vent size and patch the drywall after completion.

While I value Abe's professionalism and experience, I would take a slightly different approach.  If you are not changing anything in either room then a qualified handyperson can pull the old fan, enlarge the opening and install a new fan.  If you are remodeling the room or you are going to go from a single switch operation to a multiple switch type fan, I would then go with a remodeling contract or an electrician. 

Philip Anderson

HDR Remodeling.

Generally, you would call an electrician to replace the exhaust fans.  In our area, there are a few service companies that provide electrical services as well as HVAC and plumbing.  That way if issues arise where you need another trade, you only need the one company.  Look for a service company that has all of the services you would need for your home.

An electricain is a good option; However since the fans are already exisitng agood Handyman is probably your best bet. They normally have a jack of all trades mentality so minor HVAC and electrical work should be easy for them.