[2021] Upgrades carried over

Hello! This behaviour was added generally to encourage changing subs more freely to adapt to your changing environment, and more specifically because earlier, it was actually not always possible to switch subs below a certain depth – the upgrades that made your sub withstand the depth didn't carry over, and you died as soon as you switched to a new one, which was of course not intended. Rather than fix only that, we decided it might be simplest if all upgrades just carried over, hoping that would also encourage more players to play out entire campaigns rather than stay close to the beginning for long stretches of time.