I would recommend getting site-to-site between branches as well as that would optimise network flow. It won't bring the network down, just remove your static routes before you enable the new VPN and the router will handle the routing (sounds silly) between sites. With setting up a route through head office means you're involving your head office connections unnecessarily for traffic that could just flow directly between the branches.
If you have Exchange servers at the branches for example then you could set up connectors between the two branches and e-mails between users in the branches won't need to go through head office. Same would apply for VoIP calls if you have VoIP system, and so on.
Let us know how you get on.
