If there is data, there is data...
What you could do is this:
- extract your base data for scenarios of the type that uses that UD dimension
- reset those scenarios, clearing all data
- change dimension assignments
- reload the data and reconsolidate as necessary.
Alternative:
- create a new scenario with a different Scenario Type, using the new dimension. Remember you have the custom ScenarioType1-8, if you don't like the default names.
- copy data from the old scenario to the new, with a rule or via export/modify/import
- reset the old scenario and then delete it.