I have the new launcher and have forge installed for 1.6.2 but my son is having a fit because his favorite mods have not updated yet and wants to use the 1.5.2 version on minecraft.  We have it as a profile on the new launcher and can play it in vanilla but how do I install forge on it so he can have his mods back?

Copy %appdata%/.minecraft/versions/1.5.2

to:

%appdata%/.minecraft/versions/1.5.2-Forge

Inside this folder:

Rename from '1.5.2.jar' to '1.5.2-Forge.jar'

Rename from '1.5.2.json' to '1.5.2-Forge.json'

Install forge into 1.5.2-Forge.jar like you always would for 1.5

Edit 1.5.2-Forge.json to change the 'id' value from "1.5.2" to "1.5.2-Forge"

 

Create a new profile in the launcher, and change it's game directory to %appdata%/.minecraft/versions/1.5.2-Forge

 

Now this is hard and complicated but i'm pretty sure it'll work.

Forge for 1.5.2 came out before the new launcher existed so the install is kind a pain.

 

If you can't figure it out, then go try installing MultiMC it should be able to setup everything for you it's just a different launcher.