Does upgrading my iCloud plan extend my previous subscription?
The terms are listed deep in this link to the iCloud terms and conditions:


  The applicable storage fee for an upgraded storage plan will take
  effect immediately; downgrades to your storage plan will take effect
  on the next annual billing date.


So a literal interpretation would be you would not get any credit for the unused portion of the year you paid for the initial storage if you upsold yourself before that year term expired.

In practice, Apple has been generous with giving pro-rated refunds for AppleCare when you cancel it (but that is listed explicitly in the T&amp;C for AppleCare) and has extended MobileMe storage for free - so you could either get the lower storage tier added to the end of the higher tier (which would be a bit of a complicated thing for Apple to track - especially if you did two buys in one year).

I suppose you could open a ticket with billing support to see if they have a current policy on how they handle up-sales from one paid tier to another, but I would expect the initial purchase to end when you upgrade to a higher tier before the anniversary unless you contact Apple and they offer you a credit or extension as an exception and not something that is programmed into the system.