On the 5505 with 8.4 code, the command is:
crypto ikev1 am-disable (I would do it on both sides)
This shouldn't affect any current IKE phase 1 tunnels. The next time an IKE phase 1 tunnel is built, it should use main mode.
This command can show you the current method in use:
show crypto isakmp sa detail
I would recommend setting up a management window if you plan to test a configuration change on a production network.