Let's say you have a ContentType like this:

members: 
    name: Members
    singular_name: Member

By not specifying the slug explicitly, Bolt assumes the key of the node to be the Slug. In this case members, which will also make /members/ be the URI for the listing page. But, let's say you determine you want to have /memberlist/ for all members, and /member/mike for a single member.

You can do this by adding them like this:

members: 
    name: Members
    slug: memberlist
    singular_name: Member
    singular_slug: member

This will work as expected, with one big pitfall. Since the ContentType is now effectively called memberlist, all existing members will no longer show up in the Backend.

For now, the easiest way to fix this, is by manually updating the rows in the Database. Use your preferred Database tool, go to the bolt_content table and run the following query:

UPDATE bolt_content SET content_type="memberlist" WHERE content_type="members";

Obviously, replace memberlist with your own new ContentType slug. If you're not sure about the previous implied name, just look at the content_type column in that table for what the ole one used to be.