So if one were to want to go into the originial campagin and change the elites into cmt's elites or change the assault rifle into the h3 assault rifle and other things of that nataure what would you have to do?

You'd have to find the map you want to edit, extract the tags with HEK+, edit the correct tags with Guerilla, and recompile the map with Tool. Then if you wanted to convert it to Halo PC you'd use the program Harbinger.

Alright, it requires several downloads and what not to get started:
HEK - http://hce.halomaps.org/index.cfm?fid=411

HEK+ - http://hce.halomaps.org/index.cfm?fid=1541

Tut on HEK+ - http://hce.halomaps.org/index.cfm?fid=4503

Then, using HEK+, extract the entire tagsets of each map found in your Halo PC directory. This might be included in the tutorial. If you are unsuccessful with that, then extract the original/modified stock tags from a single player map that is not protected. I assume you know what the 'maps' folder is and what not already.

Afterwords, use Guerilla of the HEK and open the folder and .scenario of a campaign level, such as a10 (The Pillar of Autumn). Go down to weapon palette (drop-down box) and replace the assault rifle with the h3 assault rifle tags you to sub in. Next, for the elites, go down to the Actor Variants palette and do the same with CMT Elites (found in the cmt folder in the tags directory after extracting from this link: http://hce.halomaps.org/index.cfm?fid=5144). You'll need this fix of the animations for them to work properly: http://hce.halomaps.org/index.cfm?fid=5319

If you have any problems, keep asking questions in this thread.