In general, community contributors would retain their copyright to the code they contributed to the project. They license the contribution to you when they contribute the code. If you want to retain the possibility of changing the license terms in the future, you would generally need contributors to assign their copyright to you (either personally or a corporate entity you create to own the copyrights for this project) or the changed terms would need to be compatible with the new license terms. Of course, if you require this sort of copyright assignment paperwork before you can accept a contribution from the community, it is much less likely that the community will decide to contribute and you'll have to do a fair amount of work getting the legal forms in order before accepting each contribution. Plus, there is a strong chance that your project will get forked if and when you decide to change the license terms. It strikes me as unlikely that a new open source project is going to get a lot of contributions from the community under those circumstances.
It would generally be easier if you licensed the product under the split license terms initially or if the initial license terms were compatible with a future closed-source product. Code that is under the BSD license, for example, can be incorporated into a commercial product at any time so if the project and contributions are under the BSD license, you could easily release a commercial version of the same product. Your intention (or option) to produce a commercial product, though, will likely decrease interest in making contributions to your project-- most open source developers are uninterested in making unpaid contributions to a commercial product.
Of course, as with any legal issues, you'd want to talk with a lawyer rather than relying on a forum post before taking any sort of definitive action. You'll almost certainly want that lawyer to draft the copyright assignment document you'll need people to sign and you'll need to discuss your plans for the future with the lawyer to ensure that everything is set up correctly.