Your assumption seems correct to me. We've noticed that in the past when changing out legacy remotes that were utilizing the mrf-300. And yes, you can swap your existing antenna for the new narrowband version, part number is rfx-250. If you're using cat5 or similar to extend the antenna, note that the molex connector pinouts on the base and rfx differ. Also, do not use 3.5mm on one end and molex on the other. Stick with one or the other on both.