With git, one doesn't realize its importance, unless they use it. So I would suggest first use it for smaller codebase like some utility libraries or something. This will give them real exposure to git.

Don't try git with repos where lots of people are committing very regularly. Once good number of people have got their hands-on with smaller repo, then shift larger projects to git. Also then you can convince everyone with the model u want to implement.

This way you should be able to start shifting major repositories in like 2 months.