If the new field is on a system entity (account, contact, etc), then you are right, you will be able to modify it from another unmanaged solution but not from the managed solution. If the entity is a custom entity then it depends on the managed properties
that have been set (like Ayaz mentions).
Furthermore, if it is a custom entity, then modifying the entity in another solution will create a dependency between your new solution and the existing managed solution, so you will always need them both and your solution cannot live without the first managed
one.
Gonzalo | gonzaloruizcrm.blogspot.com