There's not much help with this topic. The only info you get is to be directed to this site,
It doesn't say anything about losing or not losing followers if you change your URL.
So I tested it out myself. I have 2 accounts under different email addresses. I followed Blog A using Blog B. Blog A is pretty inactive so I didn't care if I lost my followers. Once I changed the name, I did a test post. I then went to check my 'reader' with Blog B and the post did not show.
FYI, I checked the box to permanently delete the original URL on Blog A. Also this was only a URL change using WordPress.com URL's. For example, sdlfkjsd.wordpress.com to uljgbgk;j.wordpress.com. No upgrading or anything like that.
So I guess you will lose your followers if you change your URL. Unless if it takes time to update or something.
The blog I need help with is nosajnawk.wordpress.com.