If you haven't switched to determinate tomatoes this year, yes, you can gently train a main stem back down the trellis. Depending on the plant's structure and how you have pruned it, sometimes there is a sucker that is pretty well formed/mature that could take over as the main stem if you cut the top of the main stem. You might also look into how greenhouse growers train cucumbers and tomatoes to be single-stem plants on a string that they can lengthen and slide along an overhead support bar such that the plants get shifted from vertical to an angle.