updateSourceCoords and updateRefCentroids are presently static methods on lsst.meas.astrom.FitTanSipWcsTask. Given a WCS and a catalog, they update the appropriate field. They are generally useful and I propose to make them free functions that live in lsst.afw.table.
They are simple, but not completely trivial. By making them readily accessible we increase the odds that source catalogs will have correct coord fields and reference catalogs will have correct x,y centroids, e.g. after a WCS is fit. Furthermore, if it we should ever require extra speed we could code these functions in C++ and all users would benefit.