There really isn’t anything that you can do now to avoid doing the work of replacing sendMail with the new action everywhere it currently is in your rules. But the question is what do you replace it with to make it less work next time?
First let me say that Mail binding Things are all manually created. Given that, you have full control over the ID that is used. So if the Thing ID changes at any point, it’s because you chose to create/recreate the Thing with a different ID.
As for future upgrades, who can say. I doubt that there will be any significant changes to the basic Mail binding Actions in the foreseeable future.
But there are things you can do so the calls to the Mail action do not appear in so many places throughout your rules. For example, Design Pattern: Separation of Behaviors. If you are using UI generated rules in OH 3 or scripted automation in OH 2.5 you can put the calls to the Mail Action into a library function so there is only one place where you’d have to change the Thing ID should you choose at some point in the future to change the Thing ID.