It costs only 40 to upgrade Infantry (1 move) to Mech Infantry (2 moves). This turns a defensive unit into a powerful offensive unit.

It might be a good idea to garrison cities with more infantry-type units than you expect to need, with the intention of turning some of them into a Mech Infantry army late in the game. What a fine outcome for a lowly Spearman.

This is also a good argument for building spearmen (which upgrade all the way) rather than warriors (which do not) for your early barbarian hunts.

Don't overlook the cheap upgrade from Warriors to Swordsmen much earlier. Warriors can be churned out quickly, and then upgraded to the most asswupping unit of the ancient era for only 40 gold.