Good question. I don't think we have to wait for 6.0 as I don't expect this to be a big deal, and reindexing is often needed anyway.
But it is a breaking change so if we do it, it will need blog highlights, etc.. rolling it into a 6.0 would be a minor.

So it's a matter of "small consistency fix" vs "small migration issue".. I'm fairly neutral, so I'd do what's easier for us in terms of fitting within other related work. For example if it makes sense for your ES POC we could mark this for 5.5?