I think this is a stupid question, but are the numbers in the right-hand part of the advancement window cumulative? If they aren't, there are some years in some advancements that aren't worth researching.
example: Naval Air shows the same numbers for 1944 and 1945. So unless your units are gaining that number and adding from year to year, there is no point in dedicating researchers to naval air for 1945. Am I right about this? The chart just confuses me a little.
If I understand what you mean by "hold" then the answer is no. A unit can only be using the advantages of one tech level at a time. However, you can change your tech level by Retrofit as depicted above. So either the air unit is at tech close support, or Strategic Bombing. It cannot be close support AND Strategic Bombing.