What about being able to create a new XYZFrame from a old ABCFrame?

i.e.
```Python
fk5_2 =coords.FK5(ra=8*u.hour, dec=5*u.deg, equinox=J2001)

ircs2 = coords.ICRS(fk5_2)
```

Like @astrofrog said for `CoordinateRepresentation`?