Greg Maxey has a free tool that will convert legacy formfields to content controls.
Content Control Tools (search for "convert" for more about this).

Here is his general page describing Content Controls. Word Content Controls
Greg's site is my "go-to" place for information on content controls. The pages can get a bit involved because he tries to cover all sorts of possibilities, but they are much more in-depth than most sites.

See also Mapped Content Controls and Document Properties.