I'm a law student but not an expert, and I haven't researched the caselaw (yet?). Rcthompson, I apologize for taking this opportunity to go way beyond your question (see below). [This also reflects my assumption that you are coding within the US. And also this is not legal advice, merely hypothetical musing.]
Whatever you write, if copyrightable, automatically achieves copyright protection when fixed in tangible form - and you automatically obtain authorship rights. That gives you the right to grant licenses for any of the rights you've obtained (including to produce derivative works). Swift's license (https://swift.org/LICENSE.txt) allows you to modify the license of your additions (your right under federal law anyway), but not the sections you didn't author.
I mean... I can see it cutting both ways. On its face, it's a very fair system - you can use whatever you want but only own whatever you wrote. What becomes problematic is if companies (realistically) with wide distribution nets grab open-source code, modify it slightly, alter the license of the bundle and resell it. I know that's not a new phenomenon.
Another concern I see is if libraries or maybe files or something have proprietary names, then in theory you'd require an express license from the proprietor (in this case basically Apple).
I was concerned I didn't understand the basic question that prompted my response, which was from rcthompson, but he reframed it: commits 1 to n-1 have sort of Schrodinger's Cat licenses. They have copyright protection (regardless of any of this). And that grants you the right to control licensing of those rights (reproduction, public display, derivative works, etc.). But I don't see why you couldn't retroactively apply whatever you want. The Swift license applies itself, if you don't specify otherwise, upon submission of your work to the licensor (I guess per notice requirements). Until then it's unclear. However, federal copyright protections and state common and statutory laws would also apply (and would basically grant you the right to control licensing).