While this is a good answer, it makes one assumption it forgets to mention: that nobody else has made code contributions in before the license change.
If there have been contributions by someone else than you, things get more complicated. Well, you can still do all of the above if every contributor agrees.
If you don't (want to) get permission from all contributors, it suddenly depends on the original license. If that was the MIT license, for example, you can still do all of the above as long as you also include the MIT copyright and permission notices. On the other hand, if the original license was a GPL version, there's probably just about nothing you can do without permission from all contributors.
A contributor agreement can help with getting permission for changes ahead of time when people contribute code. They're not common outside of the really big projects, though.
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While this is a good answer, it makes one assumption it forgets to mention: that nobody else has made code contributions in before the license change.
If there have been contributions by someone else than you, things get more complicated. Well, you can still do all of the above if every contributor agrees.
If you don't (want to) get permission from all contributors, it suddenly depends on the original license. If that was the MIT license, for example, you can still do all of the above as long as you also include the MIT copyright and permission notices. On the other hand, if the original license was a GPL version, there's probably just about nothing you can do without permission from all contributors.
A contributor agreement can help with getting permission for changes ahead of time when people contribute code. They're not common outside of the really big projects, though.