Of course yes. If you have contracted a lower Hosting plan and later want to contract a higher one, you will only have to pay the difference between the price of the Hosting plan you want to contract and the proration of the amount you have paid for the current Hosting with the months that you have left to comply year.
You have contracted the Hosting Starter plan paying € 150. Just when you turn 6 months, you want to go to the Advanced Hosting plan. As you still have half the time left to enjoy, you would have a discount of € 75 (half of € 150) in the acquisition of the new Hosting plan, so you would only have to pay € 575.
Note: Remember that the prices shown on the web are without taxes.