It seems that you have managed to order the service before the price change of new VPS services on 1st of December 2017. Services ordered before this period had the renewal price unchanged for services. However if you try to change the billing cycle, or upgrade the service to bigger plan, the new price is calculated according to new prices that were set after the price change.
In this case you can keep renewing your service for the monthly cycle for lower price
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In this case you can keep renewing your service for the monthly cycle for lower price