if you registered you will not have to pay again if that's what you mean.
soldat gets upgraded only like once every year or two so when it does upgrade it gets very cool and new features. You can't play online with people that have different version of soldat anyways since essential game functions have been changed. Also, almost all servers will upgrade as soon as possible so you'll have hard time finding a server.
In conclusion: always upgrade, you never lose anything and new version is usually better than its previous one.