Good news and bad news .... u will find the corps prequisite tech in the unit commands xml file I changed mine from nationalism to military tradition and it worked although it won't show the change in civilopedia so keep this in mind also I found the army too and u can change the tech too but u will have to edit the civic xml file for the army and change the boost to example military training .... for this I don't recommend it cos then mobilization will have no boost . But u can change the number of the army from 3 to let's say 4 or 5 or any number u want I didn't test the army yet so My Suggestion will be as follow : Save a copy of ur original xml file Unit commands and then edit the corps by adding example MILITARY-TRADITION instead of nationalism which u won't be able to change the 2 number meaning corps are 2 units then don't change the army leave it on mobilization and save ur edited file then save a copy of civic original file and open it and look for mobilization and change the number from 3 to let's say 4 and save the edited file and that's really it .... good luck !!! And be sure to make a copy of the originals xml files before doing any editing !!!
P.s : the civilopedia won't update these changes but I can confirm that the corps works beautifully once the military tradition is researched