Elliot, you don't need to change the Miracle-Gro mix in your buckets just because of Late Blight. You might want to remove the tomato roots and add amendments to the mix next year for other reasons -- to loosen compacted mix, add organic matter such as compost, and replace depleted nutrients. But once the tops of your tomatoes have been properly disposed of or been killed by freezing weather, and provided no infected potato tubers survive the winter nearby, the Late Blight is gone too.
Late Blight does not live by itself in the soil. It does not live on plant roots. It can't come back next year from the Miracle-Gro mix in your buckets or from the tomato roots.
Late Blight will survive on living potato tubers, which are actually not roots. But it doesn't live on dead plant tissue, such as potato tubers which have been killed by being frozen solid.
You might find questions #18 and #19 helpful in this FAQ about Late Blight from Cornell:
http://www.hort.cornell.edu/departme...path/lbfaq.pdf