Absolutely required - clearly they’re not “read-only” as they can be erased (by editing Original Source) - so at least let us correct them properly.
I now have incorrect capture of data in drilldown 1 which will rear its ugly head for MONTHS in report until the period simply falls off charts - unless I make some single-use custom field + workflows to populate it and train everyone to use this duplicate field instead. The thought of managing that going forwards makes me want to hang myself.
PS: SERIOUSLY labour intensive but I suspect you actually can correct this issue by creating duplicate contacts with correct values using the Forms API and then MERGING them - but it will take tooling and manual work.