Hi Walt,
Christoph's informed point about metric is a common point of confusion - particularly for those who networked before vlan routers came along. In the old days a 100Mb routed port had a lower ospf priority/metric than a gigabit link. (a poor example since gigabit ports and switch-routers/vlan routing sort of showed up together - but it gets the job done)
So OSPF is not aware of the layer two bandwidth on the S-series. A vlan might contain many ports - ISL's, end stations etc. If you plug in the two links as described OSPF considers both to be equal cost. You would see load share behavior from the router.
There is a way to teach OSPF layer two cost but its really not worth the trouble in this case. Best bang for the buck is Christoph's direction.
In the event there is no OSPF on these links your default routes will load share given equal cost/weight on the route statements. I think its round robin algorithm, flow by flow. So configure the other default route, plug in the big pipe and traffic will automatically begin even distribution (more or less). The established flows on the 1Gb link will continue to use that path - but load will soon even out naturally.
Then cut your one gig. Or pull the route pointing in that direction. seamless transition.
you're done.
Low risk of service impact. You could do this during production - except I never recommend configuring a router - particularly a critical item like your internet egress - during production. Too many people get mad if you happen to make a mistake
Let us know how it goes.
Mike
