Just for reference, here's how
/usr/texbin winds up pointing to
/usr/texbin -> ../Library/TeX/Distributions/Programs/texbin
/Library/TeX/Distributions/Programs/texbin -> ../.DefaultTeX/Contents/Programs/texbin
/Library/TeX/Distributions/.DefaultTeX/Contents/Programs/texbin -> x86_64
/Library/TeX/Distributions/.DefaultTeX/Contents/Programs/x86_64 -> ../../../../../../../usr/local/texlive/2014/bin/x86_64-darwin
I'm pretty sure OS upgrades will wipe out local changes to the file system, excluding home directories and /usr/local. But there's enough changes that MacTeX makes in /usr, /Library, etc. that could get wiped. I've always used FixMacTeX(version).pkg after every OS upgrade, and it's always worked.