I have the task of moving a large amount of assets ~50gb from one amazon s3 bucket to another on another AWS account. I've figured out how to copy the files via S3 COPY, but I'm just wondering will it simply be a case of changing the API credentials in Craft to the new bucket? Will this retain all the assets links?
Yes that’s all you need to do. It also works the same if moving to/from a local directory or Google Cloud Storage, or even a subdirectory of any volume type. The index of file paths is relative to the root of the volume for the source, so if the volume changes it doesn’t matter.