There is the case of very interesting timbres, generally very effective and a certain limitation in extension, better refer in tessitura in those cases.
Some would think as a limitation this lack of great extension and some would like to solve it.
In my experience when the timbre is obtained by a sustain that reach the positions of second register by mimicking the first and maintaining the larynx movement of pronunciation of the first register is something truly effective and musically precious and if they do not want to absolutely sing repertoire that requires complete development and use of the three registers I would discourage to change it.
If otherwise obtained by a lowering of positions in my experience it will not damage so much the musical effectiveness to correct it.
VOCAL TECHNIQUE MADE SIMPLE
One of the frequent positions, of internal musical choice on voice that take some away from the development of their true voice is the refuse of something.
To make an example: those who totally refuse nose sound, in any way. They usually get to such a pure flat that takes away the sustain and as I said many times in my tutorials when you have not sustain you will have the sound of no sustain and in this case with correct positions they will sound truly nasal as they do not want. So avoiding the nasal resonances they are obtaining exactly to be stuck in the nasal sound any time they do not want to break the flat sound. So the correction is to put some exercises with correct nasalisation and develop sustain through that and yeah the nasal sound will go away.
Think about that anytime you are absolute in refusing a voice characteristic.