Yep, as you say, the requirement for a custom URL is now 100 subscribers.
Indeed - I think it's a step in the right direction, setting it at 500 subscribers is kind of pricing certain YouTubers out of the market. Of course, you also can't just let anyone do it because then all the URLs would get taken, so I think this strikes a reasonable balance.
You can change your name repeatedly BUT the first name you picked will always be the URL.So if you changed your name to george after first changing it to steven people will still have to search u up as george.