Follow Amazon's migration guide to update the RadCloudUpload to support version 3.x of Amazon's SDK -- it has been out for quite some time. You cannot have both v2 and v3 referenced in the same project, and Telerik forcing us to use v2 is holding back development with other items in v3. http://docs.aws.amazon.com/AWSSdkDocsNET/V3/DeveloperGuide/net-dg-migration-guide-v3.html
Yes I agree with Richard that perhaps a new provider so there would not be a breaking change.
I am stunned that this has not been done yet. I understand that changing the existing AmazonS3Provider may be a breaking change, so perhaps you should implement AmazonS3ProviderV3 that works with the AWSSDK V3 libraries.