What's your upside from changing? A longer connection and more time in the Concorde Room?

Are you trying to do (or avoid) a back-to-back on DUB-LHR afterwards, or worried you'll be penalised if you decline to fly if your connection is delayed?

If your LHR-DUB is delayed, and you have DUB-LHR on a separate booking with luggage, you can use same-day change when you land at Heathrow (or potentially, before taking of from MIA) to extend the time then.