Sell dual licensed software with 3rd party contributions

I host a project on GitHub.

Right now it has a custom license (shareware) but I'm going to move to a dual license AGPLv3 + commercial license.

I would like to have some information about the commercial part of my software:

Lot of users send Pull Requests to my repository to implement new features or fix bugs. Right now I own the copyright and all the rights of these PR. But when I'll move to AGPLv3 or later everything will be under this License.

Does it mean that when I sell my software I will be able to sell with the commercial license only the parts written by me and I will not have rights for the parts written by users?