Well you can either build the next level MIC or the same level of the new MIC type (both give new units as much as I know, but I'm not 100% sure.)
You can build the highest level MIC which gives the new units. However it will not be built cheaper or faster if you have the Highest MIC from the last "reform".
As much as I know you need to do it with all reforms - atleast for the romans.
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