Hi Konstantinos,
For virtual WSA, VLN can easily be changed. Based on my experience, even you extend your license with same VLN, you'll still need to upload new license to virtual WSA.
Since you are renewing licensing, once you get PAK with new license (in case you are using traditional licensing), you just generate new license, with blank VLN number (it will generate new VLN for you). After that, you just upload new license with loadlicense command.
BR,
Milos