Bulk change in ALL of my "form text fields"?

First, it's easy to run a script in the console window that will change all, or a group of fields.

Like this:

for(i=0;i<numFields;i++)
{
     cName = getNthFieldName(i);
     if(/*Use this if to filter names*/)
     {
           oFld = getField(cName);
           if(oFld.type == "text")
                oFld.setAction("Format","MyDocLevelFunction('test')");
     }
}

The main part of the script is the "setAction" line. It's setting the custom format script to call "MyDocLevelFunction", which is a document level script. The text string "test" is being passed into the function.   

The structure of the script is a loop that gets the name of every field on the form. It filters on both the field name and the field type. Neither of the filters is strictly necessary, but you'll need some way to figure out which fields are the ones being operated on. 

Here's a video on using the console window. 

https://www.pdfscripting.com/public/Free_Videos.cfm#JSIntro

I think this may be possible if you create a document level function and call it with a mouse-up event using a button field, an custom batch action using the Action Wizard tool, or maybe even from the JS Console.

The issue that I'm looking into is that you may also need to implement the getArray()  method.

This is easier to implement if all of your 300 text field names are prefixed. Like for example, Field.1, Field.2, Field.3, etc...

I am sure there may be paid-for tools that could work around this, and since I am learning JavaScript my suggestion may be off the mark or missing a more elaborate method.

Let's say we go with my suggestion, you'll still need to manually place  the portion of the script that will  call the document level function in each text field.