Really not sure about the premise and how much normal ranges actually change. Let's make an assumption that it is true thought and use some logic. What defines a normal range for a given biomarker? Let's say I was a really good scientist and wanted to do "better science" to find out what a healthy range for red blood cell count was. I would probably take a cross-section of people that doctors have called "healthy" and ones that they have diagnosed with a disease that affects RBC. Then I would do some nice statistics and say that 95% of people with with disease corresponding to low RBC had counts <3e12L and 95% of people with disease that correspond to elevated RBC had counts of >7e12/L. So I will define my "healthy" range as 3-7e12/L show that that corresponds to 98% of "healthy" people. Then I do some more statistics to determine false positive and negative rates, teach doctors how and when to properly utilize this knowledge (including performing the test exactly as I had) and be done.

There really is not a different way to do this. There is no equation that can tell you how many RB cells you need to have. There are some limits of upper and lower bounds, but those are not very useful, so we have to be empirical.

Now let's say I do this again, 50 years later and find that the value shifted. Is there a big problem? After all, the healthy range still corresponds to people that are "healthy" and the unhealthy range to those that have some underlying condition that doctors can diagnose. Maybe the range shifted because people in general have become "unhealthy". But then they would be diagnosed as such. It's just as likely that the range shifted because fewer people are eating lead paint, or because we decided to include people from diverse backgrounds with different genetics or environmental stimuli that were not available in the first study. Maybe the range shifted because people are much "healthier" now with more monitoring and resources available to stay healthy. As long as the range serves its purpose - identifying values that are indicative of an underlying disease - it does not matter what the absolute value is.