Is it possible to upgrade the SSD in a 2011 model Macbook Pro?
To your questions:


Is it possible to upgrade the SSD at all? I have heard rumours that they are soldered in after 2010, and are not replaceable.

Yes, it should be possible, others have done it. This video (How to Upgrade/Replace the Hard Drive in the 2011 15" MacBook Pro) shows the procedure (just for information, I understand you don't want to void your AppleCare warranty): http://www.youtube.com/watch?v=5opqLFOvh3M.
My city and country (Tallinn, Estonia) has no official Apple stores that I know of, but does have Apple-authorised resellers. Are they the appropriate place to get this done? 

Yes, I'd definitely go to an Apple authorized reseller. I could find Premium resellers in Tallinn (see https://locate.apple.com/ee/et/sales/?pt=all&amp;lat=59.43696079999999&amp;lon=24.75357459999998):

IM ARVUTID IDEAL TALLINN
NARVA MNT. STREET 7, PK 34
TALLINN EE-10117

IM ARVUTID SOLARIS
ESTONIA PST. 9
TALLINN 10143

IDEAL KRISTIINE
KRISTIINE 45
TALLINN 10615




You can select another country/language from https://locate.apple.com/country (though I used the trick explained here http://forums.ilounge.com/lounge/224663-where-can-i-buy-apple-products-sweden-finland.html).