Graphus mentions using a bushing in his comment on the OP. If you rout around the inside of the template, then a bushing will work to make your newly produced part smaller. If you rout around the outside of the template then a bushing will only serve to make the new piece larger.
If you have an "outside" template, you could use the router to create a new, "inside" template which will, by it's very nature, be larger than the original template. (Use the smallest diameter straight-cutting bit you can find to make this new template to minimize the enlargement.) Then find a large enough bushing to use your new, "inside" template to make the final piece of work the size you're after.
If you need a significant reduction such that you cannot find a large enough bushing to make the reduction in one pass, you could make a series of "inside" then "outside" templates, making each one smaller than before. When you're done, you'll have gone through a lot of plywood/hardboard/etc, but you'll have a nice set of templates that should work for just about any size you'd need in the future!