Why is a conversion between EDs from two different census years so important?
There currently is no online system for finding the 1920 ED of an address, and limited availability (mostly on the One-Step site) for finding the 1940 ED of an address. But there is for finding the 1930 ED for over 400 cities, namely the One-Step 1930 Census ED Finder. So one way of finding a person in the 1920 (1940) census is to obtain the 1930 ED for the person’s 1920 (1940) address and then use this conversion website to determine the corresponding 1920 (1940) ED. Another site for finding 1930 EDs is the NARA 1930 ED Finder. It’s much more restrictive than the One-Step site for finding EDs from city addresses, but it is very helpful in rural areas where all you have is a location. Of course this assumes that the street names and house numbers did not change between 1920 and 1930 or between 1930 and 1940. Two cities that underwent major street name changes around this time period are Baltimore Maryland and Queens New York. See question 308 on the ED Finder FAQ page for more details
