If I recall correctly then you should be able to use NetInstall to flash in random ROS version regardless of current ROS level or license level.
I tried it on a RB333 with current ROS v3.30 and sent in a ROS v5.16. As the license key format is different between those releases, it urges me to upgrade the license key by pressing "Update License Key". If I do it then I am greeted with a prompt message about "failed: could not resolve http://www.mikrotik.com". If I fire up a terminal session within the same Winbox then I can ping http://www.mikrotik.com just fine.
What I am doing wrong? The clock is ticking and I have 3 days to resolve it.