Replacing #6 Ground Wire
Interpretation of Article 250 Part III to be a little hard to keep up with, and it seems like the members of the NFPA take great pleasure in tweaking it in every edition.
Generally the paragraph that allows maximum #6 copper to &quot;Rod, Pipe, or Plate&quot; electrodes is not interpreted to include water pipe, so you would need #4 copper, but not necessarily solid.
Table 250.66 (or 250.104 if just a metal piping bonding jumper) references size of feeder to the service, which you would expect a 200A service to be fed by 2/0 copper or 4/0 aluminum for the GEC. The Table shows #4 copper or #2 aluminum, 250.62 allows stranded or solid.
A metal underground water pipe also requires a supplemental electrode.
I don't have a copy of or even know what edition of the NEC was adopted locally for your location for 1997, and don't know what exceptions may have been locally enforced.