Here's my thoughts
First and foremost as mentioned you should try to avoid invalidating any existing answers. I know you were intending not to, but I think it is worth mentioning in case other people have a similar situation in the future.
Now that that's out of the way, the main question I would like to ask is
What are the drawbacks to making the change?
The one you mention in the post is that previously competitive answers will become less competitive as new methods become available.
I think the main reason you consider this a drawback in your post (if this is wrong tell me, I don't want to put words in anyone's mouth), that the time spent optimizing the input portions of answers will go to waste. That because a new method is available to make shorter answers the time spent on them is false.
Now while this is a reasonable I think it is missing the point, there are two main things most people get out of golfing
Golfing is inherently fun and doing so is enjoyable.
Posting short answers gets you upvotes which get you reputation, which we all like.
People like competition, in particular winning them.
As far as the first one goes, a rule change can never go back in time and remove the fun you had, so no harm there. And for the second one I seriously doubt people will retract their upvotes because the rules made the solution less optimal. So on both fronts there it seems like the change has little effect.
The third is a little bit of a mixed bag. On one hand some of the most fun competitions I've had were ones where it looked like it was over but some new information provided a surge to get even better scores, and this could be that new information. But it could also be a bit frustrating having just won a competition to have to rules changed and have to defend your title. In this case it might be a FGITW to port answers to new paradigms. I would hope there wouldn't be any disingenuous behavior, but even with everyone acting fairly there might be a clear answer in a particular language and the first person to notice the change in the question gets the prize so to speak.
Now the next question to ask is
What do we gain from changing the challenge?
Now this one is pretty challenge specific as different changes might have different improvements, but the two main things I would be concerned about, are
It makes it more fun for people who answer in the future.
It might make more people able or willing to compete.
It sets a good example for future challenges.
I think all of these are pretty decent reasons to make the change, but you will have to weight them on a challenge by challenge basis.
Overall I'm not sure if I have an answer as to what you should do in every case, just some things to consider when making a choice.