Change Designation Hierarchy

We have a few designations with incorrect hierarchies that we'd like to correct. 


My understanding is that only the fundraising purpose (the lowest level of the hierarchy) is logged on the revenue and therefore it should be theoretically possible to edit the hierarchy and have all revenue logged against the purpose show under the edited hierarchy without having to change any revenue records.


However we can't seem to find a way to do that in the system. Has anyone faced the same issue and found a solution?


Any advice greatly appreciated!