Path that must be traversed to go from old thing to new thing. Can be bad in terms of overhead, like moving.
Undesirable part of getting from release 5.2 to release 6.0 - new benefits, new bugs, old stuff learned now obsolete, more stuff to learn - have to find time.
eg: hairy legs. upgrade to smooth waxed legs. migration - have hot wax applied to legs and ripped off with great force.
Something that you're very glad when its completed.