for county changes, see https://www.census.gov/geo/reference/county-changes.html
Dade County, FL is renamed Miami-Dade County, FL in 1997. For pre-2000 files, recode conum (12025=12086)
Broomfield County, CO, is created in 2001 (conum 08014). It is part of the Denver-Aurora metro (19740). It was created from parts of Adams and Jefferson counties, which are also in metro 19740, and parts of Boulder and Weld counties, which are not. There's no recoding we can do; the geo boundaries of 19740 just change a little from 2000 to ACS.
Nansemond County, VA, is combined into Suffolk in 1972. For 1970 file, recode conum (51123=51800)