I have also had this problem, but downloaded from the alternative site suggested by KMT and, fingers crossed, everything seems to be OK, although I cannot run i8.3 with el cap, but did not have scenarios problems, fortunately. The reason I was interested in the new version was to get SDK for iOS 10.3, which Apple wants my phone to update to. Having had the problem previously with missing SDK's I was wary of updating without first getting the correct files, so downloaded 8.3 and now have SDK for 10.3. However, when you say that 8.2.1 doesn't work with it anymore are you getting the missing SDK problem, or is there something else Apple have lined up for me. I have not updated my phone yet so would like to be prepared if there is some new frustrations in store, so far I have just copied and pasted missing files and use an earlier Xcode - 7.3?.
LesJay