As long as you have Calibre and your old books aren't LRX, it's not a big deal. I'm going the other way putting some Mobi books on a 505. Maybe down the line you'll add the 300 to your ebook reader collection so you can still put a reder in your jeans pocket (yes I remember the picture)
It will be fun to compare notes with you on the pros and cons of each device since I'm now playing with a K2 and 505.