These are the modifications you can safely perform to your schema without any concerns:
A field with a default value is added.
A field that was previously defined with a default value is removed.
A field's doc attribute is changed, added or removed.
A field's order attribute is changed, added or removed.
A field's default value is added, or changed.
Field or type aliases are added, or removed.
A non-union type may be changed to a union that contains only the original type, or vice-versa.