s6-rc-update is an online service database switcher: it will replace your compiled service database with another one, and adjust the live state accordingly.
Live upgrading a service database is no small feat, and no fully automated system can get it right in all cases. s6-rc-update will do its best on its own, but it lets you give it instructions to handle difficult cases; and rather than implement doubtful heuristics, it will fail with an error message in situations it really cannot solve.
To be written. s6-rc-update is currently the missing piece in the s6-rc suite, which is the reason why s6-rc has not been officially released yet. :-)