When evaluating potential community development loans what tract data should be used? Should the tract data be retroactively applied for the whole year, or should 2010 data be used through the remainder of 2017, or switch over some date in between?

There are 4 different definitions of CD activity in the CRA and only 1 depends on the tract income class. The FFIEC adopted the new ACS 5 year demographics effective at the beginning of 2017. That is the demographic data you should be using.