The only entity that can change the license of a work is the copyright holder. Hence, you would need to be the copyright holder of the entire work in order to change its license. This can be accomplished by getting any contributors to explicitly assign the copyright in their contributions to you.
However... unless you have a lot of standing in the community, the chance of most contributors doing that is pretty much zero - once copyright has been assigned to you, there's nothing stopping you from taking the code, closing it off and stopping any distribution of any future modifications you make. What value are you offering that means that I or anyone else contributing to your project take that risk?