I had a similar experience last winter, when the FRA airport was temporarily closed.

My flight to Milano was cancelled and after MY investigation, I asked the agent to fly me to Rome instead. She said: “No problem, the main priority for us, is to get everybody out of Frankfurt asap and if you want, we would even fly you to Singapore”.

As you say, it’s always better to be prepared in such situations. Usually they will book you as requested.