Some military units may be upgraded as replacements are researched.
Infantrymen may be upgraded to Dragoons at a base cost of 280 money.
Riflemen may be upgraded to Lasermen at a base cost of 240 money.
The cost is reduced if you have Military Training Sites.
You can upgrade your units
by clicking on the Military link in the left menu and then clicking Upgrade Units at the bottom.
Units must be at home when they are upgraded and take approximately half as long as training new units.
You can see the status of upgrades by clicking on the My Status link in the left menu,
then on Training Status.