SkyCoord initialization with another SkyCoord

[astrofrog]

For Quantity and Time, an implicit conversion can be made during initialization:

In [1]: from astropy import units as u

In [2]: q = 3 * u.m

In [3]: u.Quantity(q, u.km)
Out[3]: <Quantity 0.003 km>

In [8]: from astropy.time import Time

In [9]: t = Time.now()

In [10]: t
Out[10]: <Time object: scale='utc' format='datetime' value=2018-07-29 10:19:22.232158>

In [11]: Time(t, scale='tai')
Out[11]: <Time object: scale='tai' format='datetime' value=2018-07-29 10:19:59.232158>

However, this is not the case for SkyCoord:

In [4]: from astropy.coordinates import SkyCoord

In [5]: c = SkyCoord.from_name('m31')

In [6]: c
Out[6]: 
<SkyCoord (ICRS): (ra, dec) in deg
    (10.6847083, 41.26875)>

In [7]: SkyCoord(c, frame='galactic')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-7-785076b7254e> in <module>()
----> 1 SkyCoord(c, frame='galactic')

~/Dropbox/Code/Astropy/astropy/astropy/coordinates/sky_coordinate.py in __init__(self, copy, *args, **kwargs)
    223         # creating the internal self._sky_coord_frame object
    224         args = list(args)  # Make it mutable
--> 225         kwargs = self._parse_inputs(args, kwargs)
    226 
    227         frame = kwargs['frame']

~/Dropbox/Code/Astropy/astropy/astropy/coordinates/sky_coordinate.py in _parse_inputs(self, args, kwargs)
    358         # by keyword args or else get a None default.  Pop them off of kwargs
    359         # in the process.
--> 360         frame = valid_kwargs['frame'] = _get_frame(args, kwargs)
    361 
    362         # TODO: possibly remove the below.  The representation/differential

~/Dropbox/Code/Astropy/astropy/astropy/coordinates/sky_coordinate.py in _get_frame(args, kwargs)
   1823                 raise ValueError("Cannot override frame='{0}' of input coordinate with "
   1824                                  "new frame='{1}'.  Instead transform the coordinate."
-> 1825                                  .format(coord_frame_cls.__name__, frame_cls.__name__))
   1826 
   1827     frame_cls_kwargs = {}

ValueError: Cannot override frame='ICRS' of input coordinate with new frame='Galactic'.  Instead transform the coordinate.

Is there any reason not to allow this? It would be nice if this worked for consistency with Quantity and Time, and would avoid special-casing SkyCoord in the APE14 implementation of celestial coordinates.

[Cadair]

I just ran into this again, so bump!