1) Open the crusader_unitprod11 file in the main folder of MTW (it is a .txt file, so you can use wordpad or so for it).
2) Search for the units you want to mod.
3) Search for the time period you can build them. Each unit has 2 lines of code, the time period is in the first line, at about 1/3 or 1/2 of the length of a line.
4) change "EARLY, HIGH" to ALL_PERIODS (no "" needed when you don't seperate words with a comma). Be carefull not to loose any of the tabs when deleting a part of the text

You could also use gnome editor. Many people use it when modding the unitprod file, but I don't know how it exactly works (I guess I'm one of the few who don't use it)