Make header update work for SLLWCS

This doesn't work properly if SIP but 🤷

sunpy/map/mapbase.py

@@ -730,11 +730,27 @@ def wcs(self):
 
     @wcs.setter
     def wcs(self, wcs):
+        """
+        Map uses the meta dict as the source of truth.
+        When setting the wcs of the map we convert it to a header and then update the header of the map.
+        """
+        # Unwrap any wrapper classes to FITS WCS
         unwrapped, _ = unwrap_wcs_to_fitswcs(wcs)
-        header = unwrapped.to_header()
-        changed_header = dict(set(self.wcs.to_header().items()).difference(header.items()))
-        if {"CUNIT1", "CUNIT2"}.intersection(changed_header.keys()):
-            raise NotImplementedError("Sorry wcslib needs you to do more programming")
+        # Convert to a header
+        new_header = unwrapped.to_header()
+        # Reduce the new header to just the keys which differ from the current WCS
+        # We do this to figure out what's been changed post wcslib doing any
+        # conversion (such as arcsec -> deg)
+        changed_header = dict(set(new_header.items()).difference(self.wcs.to_header().items()))
+        # If any of the keys in spatial units are modified wcslib will have
+        # almost certainly changed their units to deg if they were arcsec, so we
+        # have to explicitly check this and convert them back to the original
+        # header units to not confuse people.
+        naughty_key_prefixes = {"CDELT", "CRVAL", "CD"}
+        for prefix in naughty_key_prefixes:
+            if any(k.startswith(prefix) for k in changed_header.keys()):
+                if new_header["CUNIT1"] != self.meta["CUNIT1"]:
+                    raise NotImplementedError("Sorry wcslib needs you to do more programming")
         self.meta.update(MetaDict(changed_header))
 
 # #### Miscellaneous #### #