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Can you clarify here whether 'assume' also means 'override' if present?

Yes, the value provided in the context manager will be used before any other value

The verb in the name should not be "assume", because that reads like it acts akin to a new default value. The verb should instead be either "impose" (a la the impose_finite_difference_dt() context manager) or "override".

An example of this PR's benefit. The angular separation between two 2D HCRS coordinates with different obstimes normally raises an error due to the origin shift.

>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord

>>> coord1 = SkyCoord(10*u.deg, 20*u.deg, frame='hcrs', obstime='2026-01-01 00:00:00')
>>> coord2 = SkyCoord(20*u.deg, 30*u.deg, frame='hcrs', obstime='2026-01-01 00:00:00.001')

>>> print(coord1.separation(coord2))
...
astropy.units.errors.UnitsError: The input HCRS coordinates do not have length units. This probably means you created coordinates with lat/lon but no distance.  Heliocentric<->ICRS transforms cannot function in this case because there is an origin shift.

Before this PR, one can use .replicate() to instantiate a new version of coord2 that has the same obstime as coord1.

>>> print(coord1.separation(coord2.replicate(obstime=coord1.obstime)))
13d28m54.20360928s

After this PR:

>>> with impose_frame_attributes(obstime=coord1.obstime):
...     print(coord1.separation(coord2))
13d28m54.20360928s

It's not a big advantage for this simple example, but imagine working with a whole collection of coordinates or working with implicit coordinates (e.g., reprojection).