[darkpuma]

I think switching once you're an established swimmer is going to be virtually impossible, at least if you want to be competitive. I think it's something that should be trained as early as possible. If any established Olympic swimmer tries to make the change, my guess is they won't be swimming at an Olympic level any longer, because breathing on a single side is a pretty good local maxima (only marginally worse than bilateral) but getting out of that local maxima would involve degrading your performance for however long it takes for bilateral swimming to feel just as natural, which might take years for all I know. I don't know if there are any top-tier swimmers who've ever made the switch and remained competitive, I've not looked into it that far, but I suspect there aren't.

I also belief (with no scientific evidence) that training bilateral from the beginning is even more advantageous for mediocre to poor swimmers, since the symmetry in breathing will make their form in general more symmetric and consequently more efficient. Obviously bilateral breathing is not necessary to achieve excellent form, as demonstrated by the vast majority of Olympic swimmers, but I believe it should make it easier for the common swimmer.

[dredmorbius]

I switched from strong-side to alternating late in my training (17, had been swimming for 10 years). It was awkward for a few weeks, but became comfortable. My times improved through this period.