Tails in cavities

[hightower]

See my crudely drawn picture below. Red line is the current tail run (as far as I can tell) and it runs in the cavity of the wall. Currently in 16mm with a 4mm! earthing conductor, TNCS. There is no isolator between the meter and the CCU, and the meter is a new smart meter, 80A fuse at the head (or at least that's what the sticker says on it).

So I have a few questions on a CCU upgrade:

1) 16mm is fine for an 80A cutout, so first question is would you even upgrade the tails or leave the existing ones in place should they seem okay?
2) If you would opt to put new tails in, how would you go about it:
a) pull the new tails through in the same run as you pull the old ones out?
b) pull a SWA through the cavity
c) run a SWA visibly on the external wall and then go through at 90 degrees behind the consumer unit
d) something else entirely
3) The earthing conductor would need bringing up to date, would you pull this in and terminate in the head yourself, pull it in and phone the DNO up to terminate, or something else entirely.
4) If you opted to upgrade the tails would you upgrade all the way back to the meter, or fit an isolator and run the old tails to the isolator and the new tails from the load side of the isolator?

Look forward to your thoughts on this.

Thanks