Simple open-source game first, similar commercial game later. Allowed?

You own the rights. You can license it under whatever rights you want.

You can give it away under one set of rights, then later sell another version under a different set of rights.

What you cannot do is give it away under one set of open-source rights, then later revoke those rights. Once they are granted to the public, that release will forever be public.

Also if the public contributes back in, you must follow the rights that they grant; if they contribute back you cannot later change the license on their stuff unless they specifically grant that in the license.

Thank you very much!

Also if the public contributes back in, you must follow the rights that they grant;

What happens if I simply let someone make a change in the code, and they say nothing about it they just "do" it. Then perhaps that code is just as useful in the future commercial product so I use it. Do I have to be wary of that? Perhaps they meant it to be free to use, but what if they wanted something if it became commercial but didn't say anything? Is it a written agreement or a contract where that only applies?

Thank you very much!

[quote name='frob' timestamp='1335921563' post='4936618']
Also if the public contributes back in, you must follow the rights that they grant;

What happens if I simply let someone make a change in the code, and they say nothing about it they just "do" it. Then perhaps that code is just as useful in the future commercial product so I use it. Do I have to be wary of that? Perhaps they meant it to be free to use, but what if they wanted something if it became commercial but didn't say anything? Is it a written agreement or a contract where that only applies?
[/quote]

The person making the changes holds the copyright to those changes and you're only allowed to use them if they grant you a license to do so.

You could get a lawyer to write you a suitable license but you'll most likely not get a lot of contributions if you require contributors to grant you rights that you didn't grant them.

The GPL is successful because contributors have the same rights as the one receiving the contributions (You can do with their code the exact same things as they can do with your code, no more, no less).

more permissive licenses such as BSD or WTFPL might be a better option. With these licenses you give the users alot of rights and its up to them if they want to give you any rights at all in return, While you probably won't get as many contributions as with a GPL licensed project and you'll also get proprietary forks competing with you the contributions you do get can be used as you see fit.

Yes, stay away from GPL (or any copy-left licences) if you want to be able to incorporate the code into a closed-source project later. Look up licences such as the MIT or other permissive licences.

As Simon pointed out, rolling with a permissive licence also grants anyone, not just you, the right to use your code in a closed-source project.