This study compared Adventist vegetarians to Adventist non-vegetarians. Adventist vegetarians lived 1.5 years longer than Adventist non-vegetarians.

The gain from SAD to WFPB is even greater because:

1. WFPB is healthier than vegetarian.
2. vegetarian is healthier than non-vegetarian (1.5 year gain)
3. non-vegetarian Adventist diet is healthier than SAD.

So by addition, the gain from SAD to WFPB should greater than 1.5 years.

Ten Years of Life: Is it a matter of choice? 2001
https://www.ncbi.nlm.nih.gov/pubmed/11434797
In table 3, Covariates at Medium Risk, Vegetarian to Nonvegetarian, Men 1.53 years, Women 1.51 years.

Evaluation of an Eight-Week Whole-Food Plant-Based Lifestyle Modification Program, 2019
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6770132/

• There were 79 participants, all self-referred from the community, including 24 (30.4%) who were already vegetarian or vegan at baseline… eight-week group program utilizing an ad-libitum whole-food plant-based dietary pattern.
• Participants experienced statistically significant reductions for BMI, blood pressure, and cholesterol.
• When comparing vegetarian or vegan participants to non-vegetarian participants, there were no statistically significant difference in reductions for BMI, blood pressure, or cholesterol.
• Despite the general perception of healthfulness, vegetarian and vegan diets do not exclude processed foods and as a consequence, do not differ much in macronutrient composition from non-vegetarian diets. In the Adventist-Health-Study-2, the percent calories from fat in vegetarian (lacto ovo vegetarian) and vegan (strict vegetarian) diets did not differ significantly from non-vegetarian diets. Strict vegetarians consumed a mean of 29.8% of their calories from fat and lacto ovo vegetarians consumed 33.1% of their calories from fat compared to 35.1% in non-vegetarians. By contrast, the plant-based diets shown to reverse heart disease contained approximately 10% calories from fat. This very low-fat content was achieved by exclusion of added pure fats, processed foods, and high-fat plant foods.

Association Between Dietary Fiber and Lower Risk of All-Cause Mortality: A Meta-Analysis of Cohort Studies, 2015
https://academic.oup.com/aje/article/181/2/83/2739206
When comparing persons with dietary fiber intakes in the top tertile with persons whose intakes were in the bottom tertile, we found an inverse association between fiber intake and all-cause mortality, with an overall relative risk of 0.84. There was a 10% reduction in risk for per each 10-g/day increase in fiber intake.

https://nutritionfacts.org/video/dietary-guidelines-from-dairies-to-berries/
Finland dietary guidelines greatly reduced cardiovascular and cancer mortality, leading to greater life expectancy: approximately 7 years for men and 6 years for women.

Successful prevention of non-communicable diseases: 25 year experiences with NorthKarelia Project in Finland, 2002.
https://www.who.int/chp/media/en/north_karelia_successful_ncd_prevention.pdf
The dietary changes led to about 17% reduction in the mean serum cholesterol level of the population. At the same time, however, smoking in women some what increased, but from a low level. Leading to greater life expectancy: approximately 6 years for women. The general health status of the people has greatly improved. The reduction in serum cholesterol level has been the strongest contributor.

Introduction to Finnish NCD Prevention by National Institute for Health and Welfare
https://thl.fi/documents/189940/258977/Introduction%20to%20Finnish%20NCD%20Prevention.NCD%20seminar.Helsinki%2014032011.pdf

• page 23, Observed and Predicted Declines in Coronary Mortality in Eastern Finland, Men
  The graph shows most of the decline in Coronary Mortality was predicted. [So about a 1 year of increase in life expectancy was due to diet change.]