New Connecticut counties, er... regions

This regards those new Connecticut "county equivalents" or "planning regions."
My apologies if this issue has already been covered and I overlooked it.
www.federalregister.gov/.../change-to-county-equivalents-in-the-state-of-connecticut

Some users have helpfully pointed out how data for the new areas can be aggregated from lower-level geographies, such as county subdivisions.
Which is fine, if one has the data at those more detailed geo levels.

But what about the wealth of useful data published at the county levels (such as health indicators)?  How might we convert new data for the "county equivalents" back to the previous CT county structure so it can be used with pre-existing data for analysis, comparison, reporting, trends, etc.  We don't want to leave a big hole for the state of Connecticut until all agencies and companies switch over to the new counties in a year or two.
Or when we switch over to the new CT regions to include in our list of all US counties, how should we convert data for the earlier CT counties to its new CT equivalent geographies?

The only workable solution I can imagine to convert new CT counties to previous counties (and vice versa) is to create an allocation table which shows the percent of each existing CT county in the new regions/equivalents.  The value for allocation would be either area or population.

Anyone have a better idea?  Would this approach give "good enough" estimates of the new counties?

Thanks,
Bert Sperling
bestplaces.net