Is there any way to change game pace of a game already in progress? I started playing on epic and then got tired of how much it takes to build something, research etc. I would like to change game pace from epic to standard without starting a new game.
It is not possible to change the game pace once the game has already begun.
This article details the mathematics behind these mechanics and details how they're effected in the various game speeds.
This thread from the Steam forums details the effects the game speed has in less mathematical terms and also includes information on how map sizes effects gameplay as well.
In addition, the game speed also affects the number of turns in each game.
Game speed effects a fairly substantial number of the mechanics in the game, including but not limited to;
For example; (copied from the Steam link)
Game speed: QUICK
Game speed: STANDARD
Game speed: EPIC
Game speed: MARATHON
Product cost = Cost (In Hammers) to produce a WARRIOR.