You mean Johnson and Clinton, those were the major changes in definition, in 1967 and 1994.
Prior to 1940 there were no coherent statistics on Unemployment. As far as I can tell there were no surveys in the 1920's, some supplementary questions in the 1930 census (I could be mistaken on that), but the first real attempt was a mail survey in 1937, followed by the first sample survey of the population in 1939, leading to the CPS in 1940. All "data" on Unemployment in the '20's and '30's are rough estimates.
Prior to 1937 the concept of unemployment was different. It was in the 1937 survey that the "activity concept" was first used
The activity concept refers to a person's labor market status being determined by what he or she was doing during a specified time (which has come to be a specific week). Prior to the 1937 postcard census, the labor force concept used was the "gainful-worker" measure. This referred to persons who were reporting themselves as having an occupation from which they had earned money or a money equivalent, or in which they had assisted in the production of goods, regardless of whether they had worked or looked for work at the time of the census.'
(
http://www.bls.gov/opub/mlr/1984/06/art2full.pdf">The Current Population Survey: a historical perspective and BLS' role) So a lot of people who at the time did not have jobs were considered employed because they defined themselves as having an occupation.
The first survey had no specific questions, though. In 1945 4 questions on economic activity were added.
In 1957 people who were on layoff but expected to return in 30 days and those waiting to start a new job in 30 days were reclassified from employed to unemployed.
The definition of Unemployed until 1967 was:
...all persons who did not work at all during the survey week and were looking for work, regardless of whether or not they were eligible for unemployment insurance. Also included as unemployed are those who did not work at all and (a) were waiting to be called back to a job from which they had been laid off; or (b) were waiting to report to a new wage or salary job within 30 days (and were not in school during the survey week); or (c) would have been looking for work except that they were temporarily ill or believed no work was available in their line of work or in the community.
The problem with this is that there was no period specified of when the person last looked for work, what they did to look, or if they could start a job if offerred. Under Kennedy a Commission was formed, and its recommendations were implemented in 1967 under Johnson, defining Unemployed as:
Unemployed persons comprise all persons who did not work during the survey week, who made specific efforts to find a job within the past 4 weeks, and who were available for work during the survey week (except for temporary illness). Also included as unemployed are those who did not work at all, were available for work, and (a) were waiting to be called back to a job from which they had been laid off; or (b) were waiting to report to a new wage or salary job within 30 days.
The ONLY change to that definition was in 1994 when people waiting to start a new job had to have actively looked in the previous four weeks to be counted as Unemployed.
So much for the nonsense about the U6 being more historically accurate.
Under Reagan, the only major change was that for a few years there were two versions of the official rate (then called the U5), one of which included military personel stationed domestically as part of the labor force and the other which didn't.
The definition of Unemployed did NOT change under Clinton, but the definition of "Discouraged Worker", (part of "Not in the Labor Force") was changed to require having looked in the last year. A new category of "Marginally Attached" defined as all those who haven't looked for work in the last 4 weeks (for whatever reason) but who had looked in the last year and are currently available. "Discouraged Workers" are a sub-section of the Marginally Attached who didn't look because they didn't believe they could find a job.