merging styles?

It appears that what you ask is possible using thes commands:

CharacterStyle.applyTo (textItem:any, clearingOverrides: Boolean )

This one will work by you having to go through all the text frames and their characters to edit the character's style. This is not great because there's no such thing as "select same character style", so you'd have to go through everything in a loop, but it is good because it leaves your existing Character Styles alone.

Next, the CharacterStyle object has a characterAttributes property, so you can edit an existing character style, a change to which should propagate through all your textframes. This is good because it is super-efficient but bad because you're changing the makeup of your character style. However, if you're using suffixes and have a naming convention, this wouldn't be a bad thing since you'll just transform one style completely into another one with name and all.

Here's an example of where changing a style itself will have an effect on the document contents (RGB document only) :

#target illustrator

function test(){

  var doc = app.activeDocument;

  var s = doc.characterStyles[1]; // Character Style 1

  var clr = new RGBColor();

  clr.red = 255;

  s.characterAttributes.fillColor = clr;

};

test();