Yes, you can do that without having to reinstall your server, as many times you want. All the information on your server will remain intact. Open the Servers tab, select your server and click on 'Server size' You can then use the button ‘Change’ to modify your server. Just follow the steps indicated. Make sure you only click 'Next step' at the Software selection, since choosing software here will reinstall your server. It's possible to increase and decrease the server size.
Watch out, there is one exception. When you decrease the HDD size of your server we have to do a fresh install, due to technical limitations. You can increase HDD size without the need to reinstall your server.
We will correct the adjustments you've made on your next invoice.