Here's a quick way to do it, and even though a hex editor will be involved, you can actually edit the text with XSE:

1. First, open up your rom with a hex editor. Make sure you have CHAR enabled instead of HEX. That way you can search actual letters, not numbers. Also make sure you have a table file ready.

2. Search the first line of Prof Oak/Birch's text, and once you find it, write down the offset. Do this for every sentence/string.

3. Open up AdvanceMap (make sure XSE is set up with it), then create a person event. Create a generic talking script for the new person event, but use the offsets you wrote down earlier as the text strings.

4. Now if you talk to that person in-game, you'll notice that he/she will say exactly what Prof Oak/Birch does in the intro. Well, with XSE, you can easily edit it from there like any other talking script, without annoying hex editing! Even though you're editing it from a new person event, it will still affect the intro text.

5. Once you get everything working, just delete the person event.