Your VPS plan can be customised at any time - memory, storage and network transfer can be modified from our control panel on demand right here -

You are only billed for the resources you use for as long as you are using them - for example, if you increase your memory over the weekend, you'll only be billed for the extra usage for that time period. 

Additional CPU cores can only be purchased in tandem with memory. To increase the number of CPU cores, drag the combined memory+cores slider to the right.

Changing memory, CPU cores, and disk space requires a reboot of the VPS to effect the change. The IP address, configurations, and contents of the disk remain intact.