Technically, both the original Open Source Definition https://opensource.org/osd and the older Free Software Definition https://www.gnu.org/philosophy/free-sw.html.en#translations both explicitly require the ability to use programs in a commercial context.

In the case of the Free Software Definition, this makes perfect sense: the FSF wants people to have certain freedoms at work, as well as freedom in their personal lives.

The other challenge is that turning an open source project into a proprietary project isn't as simple as changing the license and asking for money. You need to build an entire business from scratch, and learn to close sales somehow (or sell to consumers). This will take up at least half your time.