There are no ways to change the triggers for the vanilla Marian Reforms. They are hardcoded to occur when a huge city is built by the Romans in a settlement with the "Italy" hidden resource (and possibly a couple other minor requirements). The only other option would be to make a scripted reform for all the units (though that wouldn't work for bodyguards) and there aren't enough buildings for more reforms, especially if they are only for a single unit.