Do you upgrade obsolete units?

I'm now facing an upcoming war that will start very soon and although I'm able to build rifleman, all my units are pretorians or archers. I would be able to gradually upgrade them but it would cost large amount of money and my research would obviously suffer. Is it wise to put so much money into upgrade or is it better to build riflemen from scratch and risk having inferior units at the start of the war?

I mean, economically, does it make sense to use upgrade function at all?