To be honest, a designers creed may be different than a doctors creed of "do no harm" because in gca, "do no harm" is more in the eye of the beholder.
If we use the scientific process/design process of identifying a problem (if it exists) and proposing solution(s) that solves most or all of the problems noted, and picking the one that seems to solve them best, then we are doing our job, regardless of if its a restoration, renovation, or sympathetic renovation.
As TD notes suggests, it is often best to go in with a mindset to make the least amount of change possible, and this kind of design process idea should ideally lead to the most minimal changes possible. In other words, look at every feature and if there is no problem, don't try to solve it.
And, in truth, I have found the problem based scenario the easiest to sell at most courses, although a few have ID'd their problem as a need for a signifigant new look to attract business, which then leads to a total blow out. Or a general need to improve the infrastructure to improve maintenance to 21st century standards. (i.e. improved drainage, irrigation, greens construction, bunker liners)
There are always some interesting problems that may change the original architects intent and affect what we do. Usually, its the addition of trees and cart paths that cause shade and circulation problems that are simply different than when they designed the coruse originally. Of course, in other cases, its turf types, green speeds, increased play (requiring bigger tees) additions (perhaps the practice range originally left out, or a road realignment, etc.) And most often these days - desire for better bunker conditions, leading to the question of "Is it better NOW to have flash bunkers, or flat bunkers that won't wash, regardless of former style?
Besides all of that, most clubs that have survived have looked forward to future members, and not back. Surely, sometimes, the history plays into their attractiveness in the market place to new and existing members. In other cases, the clubs are just as proud of their tradition of modernizing and setting new standards of maintenance, facilities etc. for their current dues paying members. So, within the narrow prism of gca.com, not all are of the same ilk, and there is a lot to be said at most clubs for "improving it" in the name of changing it up to a new look, just as sometimes its nice to paint the living room and change the furniture. Short version: Not every club is classic of golf course design that cannot be improved, but those are the ones we focus on.