For example, I want to schedule a scenario to execute with 100VU and then afterwards have the same scenario scheduled for 50VU?
Or do I need to create the same scenario several times with the different loadings?
Running different simulated users means that it's different load test settings so it makes better sense to create multiple scenario with different load policies for each.
That way you will be able to run them one after the other.
The most flexible way should be to define several population and use different load settings within the same scenario. In that situation you have the choice between playing the different load at the same time or chained. For chained: see the start policy of the Population in the scenario panel.