How many years that the users wish to copy are we talking about? And what is the alternative currently to using a new scenario?
First I would say just adjust your reports manually. Probably faster than migrating all the data. But I assume you discussed that already in lengthy detail.
Then, why not use a new scenario and e.g. a dynamic UD8 member for reporting that displays data from the history scenario prior to 2024 and current scenario data from the switch onwards? No copy needed and only a single dynamic UD8 reporting member that needs to be implemented and that the users need to use for reports that span across this "switch".
Copying is also possible if one really wants to invest time into that. Maybe with Cube Data Manager from the OneStream Openplace that does not take that long. Or use a hybrid (not dynamic) scenario for that to combine the data into a single scenario for reporting.