Replacing crowns with posts?

Hi My front 4 top teeth are crowns with posts, I recently visited a dentist and mentioned about having them replaced as They are 20 years old now and look ugly and discoloured. she said its quite risky to replace crowns with posts as i could end up loosing the tooth or its not suitable for a new crown so would need implant, would this only be known once the old crown was removed? then leaving me with no other option but implant? thanks