When you upgrade your subscription (from a monthly to a yearly subscription), you will be charged for the yearly subscription minus what you have already paid for your monthly subscription (pro rata calculus). 

When you downgrade your subscription (from a yearly to a monthly subscription), your account will be balanced with the time you didn't use with your yearly subscription.
For example: you subscribe for a yearly subscription in January (so you pay €150) but you change it to a monthly subscription at the end of May (don't forget that the price is different for a yearly (€12,50/month) and a monthly (€15/month) subscription). You used 5 months of your yearly subscription, which means you used €62,50 (5*€12,50) out of €150. So your account will be credited with €87,50 (150-62,50). This amount represents almost 6 months (5,8*€15 = €87) in a monthly subscription, which means you won't pay anything during 6 months.

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