Switching Weapons

Is there any way to make a unit switch from primary to secondary weapons or vice-versa (I.e. make your phlangites switch from sarissa to sword)?

It depends on the type of unit. Melee cavalry can be switched to alternative weapons (which are better for close combat) by simply alt-right clicking on the target.

It is also easy to switch phalanxes. Just take the pikemen out of phalanx mode and they'll fight with their short swords.

Regular infantry, though, present problems. The few infantry that do have two melee weapons generally have one high attack spear and one high lethality longsword or an AP weapon. You really can't control what they use short of removing one from the EDU files. I've heard that they will respond to attacks with their secondary weapons and initiate attacks with their spears, though I have not resolved this to my own satisfaction.

press down alt-key while clicking at the enemy you wish to attack with the secondary weapon.

This sort of stuff is in all the Vanilla game manuals. Check them first.

though sometimes I've seen units such as the Kardaka or other hoplite-like units switch to their secondary weapons when charging. it's probably automatic sometimes such as when missile units get engaged in melee.

Certain units have swords as their primary weapon and spears as their secondary

Charging with the wrong weapon is a RTW bug and the reason many units like Hoplitai only have spears and no secondary weapon.