Lloyd,
I agree with you that it is highly unlikely that any architect had exactly the same ideas at 35, 55, and 75. But in fact, many golf architects of the pre-WWII era didn't seem to live near the average life span. Few seemed to live to 60. Whether due to the bottle or the business, it is hard to say. What I'd like to figure out is what architect(s) seemed to change the most over time.
Honestly, when I think of the relatively limited design output of Flynn over a period of 35 years, I am hard pressed to think of the ways he evolved his design practices. He was showing elements, as Tom Paul has observed, of being a transition architect, that is bridging the classic age with the modern age by utilizing elements of the ground game and aerial game within courses pretty early on in his career.
Certainly one design area of Flynn and others that did change and must have reflected a sympathetic culture that allowed a different view of golf course design was the development of championship courses. These were designed as serious shot testing courses that weren't for everybody. The best and a very early example of this is at Pine Valley begun in 1912 and finished in 1921. Flynn started building such courses including Huntingdon Valley (1927) among others in the 1920s through the 1930s with his greatest design at Shinnecock Hills in 1931. The stock market crash, WWII, and Flynn's death in 1945 did not allow us a chance to see what he would've done in post-WWII America. As other classic golf architects passed away or stopped designing, it is interesting to imagine what directions golf would've taken had Flynn lived and worked another ten years or so.
Flynn's earliest designs, such as Kilcare GC in Heartwellville, VT (begun in 1909) and Doylestown CC (1916) were fairly rudimentary with squarish greens and repetitive bunker designs around the greens. However, the CC of Harrisburg (1916) and Eagles Mere (1917) look far more sophisticated. Flynn's designs from 1920 onward showed an amazing consistancy in excellence. Early on he was a daring router of golf courses, used perception and psychology in his designs to lead and mislead golfers. Likewise, Flynn at an early stage and throughout his career used natural lines and where there was a lot of engineering, made the sites look natural with great care and expense. Flynn believed natural lines were not only more appealing to the eye and to play but also that they would be less expensive to maintain over time.
An artifact of Flynn's method of operation that needs to be taken into account in an analysis of design changes over time is that on many projects he continuously "improved" the design and made changes on individual courses, sometimes over long periods of time. Merion was constantly being worked on from its opening in 1912. It was in 1934 that significant changes seemed to come to a halt. Cascades (opened 1923) was tinkered with yearly through at least 1935. There were bunker revisions at Rolling Green begun just after its 1926 opening and on through the 1930s. These changes make it hard to see how Flynn's designs were evolving since on numerous sites he went back and made revisions to represent his most contemporary philosophy.