It's always possible, but it would make no sense for it to do so. Lemmy runs entirely off donations, and it's a free and open source product first (looking at how the code is being written, how the organization is structured, and the fact that the open source community is building it as opposed to employees at a company etc.).

With Lemmy, there are many different groups (and individuals) running their own instances. Lemmy is the program that is running on the server, and when there's an update with new features, everyone downloads the new version. If Lemmy were to go closed source, I'm sure that the open source community would just make a fork and continue working there, and most (if not all) of those instances would just download that version instead.