You might want to look at DMVPN, this is the preferred method for doing hub-to-spoke topologies with routing over them. You configure only one tunnel on the hub, and you don't need to keep changing it each time you add a new spoke. It works for dynamically-addressed spokes. It also dynamically builds spoke-to-spoke tunnels directly as needed, taking some of the load off the hub.
There is an excellent document on it here:
http://www.cisco.com/warp/public/105/dmvpn.html
It's quite long but well worth the read. There are many, many advatanges to a DMVPN setup over a standard GRE/IPsec config, it'll be a bit of work to change all your configs but it'll be well worth it in the end, particularly if you're planning on adding more spokes in future.