I know this is an older thread, but this may come in handy for others looking for a way to modify their datafeeds.
For a file that large, you probably want to use something
other than Notepad or Wordpad because it will cause those programs to hang or take forever to open, plus it's much easier to make a mistake with editing.
Instead, you should probably use Microsoft Excel (or OpenOffice Calc if you don't already have MS Office). When the file is imported, you can use the Excel Find/Replace function which is usually pretty fast to get through them all and you can even use it to
sort your datafeed for easier modifications.
Also, I just finished up a short (7 page) guide on preparing a datafeed for use. I actually decided to whip it together while I was modifying a datafeed to use on one of my sites. For the walk through, I'm using a datafeed from Sharasale and MS Excel for editing; it has plenty of screen shots so you can see what I'm doing.
You're welcome to download it if you like:
Datafeed Preparation Guide
It was written mainly for WPDFI users, however, I believe
anyone working with datafeeds and wanting to learn more about formatting can benefit from it.
Hope it helps,
~ Teli