Here is an example of what needs to be altered to change the prereq techs for unique units for better role playing with an Eras Mod. This only requires Notepad to alter files.

File: [Mod Location]/High Elves/XML/High_Elves_Units.xml
Change all instances of "<Class>UNITCLASS_LANCER</Class>" to "<Class>UNITCLASS_KNIGHT</Class>"
Change all instance of "<PrereqTech>TECH_METALLURGY</PrereqTech>" to "<PrereqTech>TECH_CHIVALRY</PrereqTech>"

You may also want to change the <Combat> value and/or the <Cost> & <FaithCost> variables to reflect the earlier unit.

File: [Mod Location]/High Elves/XML/Civ_High_Elves.xml
Change all instances of Change all instances of "<Class>UNITCLASS_LANCER</Class>" to "<Class>UNITCLASS_KNIGHT</Class>"