Just don't forget to adjust the phase2 network selectors appropreately like [0/0<->local subnets] or back to the default [0/0<->0/0] if you have configured specific ones already.
I'm having the same problem, I have several branches with 50E fortigate and in the HQ a 80E fortigate, I want branch internet traffic to go through the VPN tunnel and exit through the WAN of the HQ. Did you solve this problem?