revert & allow convertion from actual type to alias

edb/schema/expraliases.py

@@ -304,8 +304,16 @@ def _create_begin(
 
             # generated types might conflict with existing types
             if other_obj := schema.get(alias_name, default=None):
-                vn = other_obj.get_verbosename(schema, with_parent=True)
-                raise errors.SchemaError(f'{vn} already exists')
+                # special case:
+                # conversion from an actual type into an alias is allowed
+                allowed = (
+                    isinstance(other_obj, s_types.Type) and
+                    not other_obj.get_from_alias(schema)
+                )
+
+                if not allowed:
+                    vn = other_obj.get_verbosename(schema, with_parent=True)
+                    raise errors.SchemaError(f'{vn} already exists')
 
             type_cmd, type_shell, expr, created_types = self._handle_alias_op(
                 expr=self.get_attribute_value('expr'),

edb/schema/types.py

@@ -106,10 +106,7 @@ class Type(
     # schema without revealing weird internals.
     from_alias = so.SchemaField(
         bool,
-        default=False,
-        # cannot alter type from being produced by an alias into an actual type
-        compcoef=0.0
-    )
+        default=False, compcoef=1.0)
 
     # True when from a global. The purpose of this is to ensure that
     # the types from globals and aliases can't be migrated between