I am currently facing an issue where an external system passes back an 18-digit Id that our platform has previously sent to them. However in the process the capitalization of the Id has been removed - read everything has become uppercase. I would imagine somebody has faced this problem before, but apparantly this is not the case.
So in short; is somebody aware of any existing (apex-) function that converts uppercased 18-digit Ids to a valid Salesforce Id?
Please beware; while a lot of answers exist for converting 15 to 18 digits, which is trivial, this is a different problem.