You can upgrade or downgrade your hosting package at any time.
The best way to do this would be to pay any existing invoice when due, then go to https://aegishosting.co.uk/support/clientarea.php?action=products
Click the icon in the right hand "Details" column in the row with your domain in to show your current package details
Click "Upgrade/Downgrade Package"
Choose the package you want eg. XTR - £100 Annually.
That way the accounts are all updated automatically and you get charged just for the upgrade pro rata. It saves a lot of manual fiddling for us this end and allows you to upgrade or downgrade your hosting package at any time to suit your needs.