If you are the sole copyright holder (i.e., the owner), you can do anything you want with the code, including doing a derivative version of the code where the only change is to the license. Licenses are just descriptions of the conditions placed by the owner(s) on the non-owning users of the code. They do not constrain the owner.
When there is multiple ownership, things get more complex (formally, all copyright holders have to agree in order to change the license). There's a gray area about what sort of contribution would be required by someone for them to be a copyright holder; its almost certainly not done by mechanical count of lines modified as a substantive contribution could be very short and a non-substantive one very long (e.g., converting all the indentation to tabs or spaces). We can't assess the extent to which this applies in your situation, except to point out that someone else downloading and using the code doesn't obligate you to grant them ownership rights.
If you write all of it yourself, you don't need to pay much attention at all to the complexity in the previous paragraph. You can just go ahead and do what you want to do.
A separate point is if you are working for a company that is the owner of the code. In that case, it's the company's decision and you're just acting on the company's behalf. It's no more complex than before provided the company is the sole owner of the code.