Yes, it usually can't be done with one massive checkin. First get buy-in that the old pattern will be changed to the new pattern (hopefully you won't have some senior developer who likes it the old way and fights you), then as you say, come up with a plan to get it done.
The down side to this that I've experienced more than once, though, is incomplete conversions: we thought we had agreement that the change should be done, it turns out to be more difficult than planned, it gets partially completed and then management has a new fire for us to fight, resources are taken away, so you still have two or more ways of doing things.
The down side to this that I've experienced more than once, though, is incomplete conversions: we thought we had agreement that the change should be done, it turns out to be more difficult than planned, it gets partially completed and then management has a new fire for us to fight, resources are taken away, so you still have two or more ways of doing things.