You can upgrade to a bigger webhosting or reseller hosting package at any time. It isn't possible however to switch between a webhosting and reseller hosting package.
To upgrade, you can login to your customer panel at https://www.versio.uk/customer.
Once logged in, you can look up your webhosting or reseller hosting account by first clicking 'Your webhosting packages' or 'Your reseller hosting packages' on your left'. Then click the magnifying glass behind the relevant product and click Upgrade. You'll be shown the possibilities with the associated price.
An upgrade is performed real-time, meaning that you can instantly enjoy the bigger package.