replacing a staircase

[cookie1000]

We have a split level property and want to replace our existing 1970's stairs. You enter at the top of the house into the hallway which is basically a 'landing' (3 newel posts with solid pnales in between, a 90 degree turn and a newel which has been half sunken into a stud wall. We have looked at buying new spindles, baserails and handrails but is it possible to fit these into existing newels??
If we needed to replace the whole thing ie newels included, how easy is this??

 

[noseall]

The uppermost part of some newel posts can be replaced, i.e. those posts that do not go on to support stair elements above. There must be plenty of the base of the original remaining though as to not weaken the stringer joint.

Any new additions must be adequately glued and dowelled into place. The joint needs to be secure and without play as it still supports the handrail and spindles.