Q: If I change my iCloud 10GB plan to the 20/50GB plan mid-year, will my cheaper plan be partially refunded or will the second plan be automatically discounted? I don't want to be paying extra...

Apple's documentation doesn't explain what will happen, and iCloud hasn't been going long enough for this sort of detail to emerge in posts here that I've seen. However I cannot imagine for a moment that you would be charged double. Either the unused portion of your original purchase would be credited against the upgraded subscription, which would then renew a year after you'd done this; or you would be charged the proportion of the extra cost of the upgrade for the remaining part of the subscription and it would be renewed on the same date as before. Either way you won't lose money on it.

I had this exact issue, I had the $20.00 plan and then a month or so into the plan year, I upgraded to the $40 plan and you would think I would receive a pro-rated charge for the $20.00 I had already spent, but I did not. I just talked to Apple and while it took a few people to obtain an answer, I did. They told me that I will recevie a credit on my credit card, but it takes between 7-10 days to process. Wil see what happens and report back.