It appears you have a couple questions.
First, baseline data is stored at the task level. You can add/remove/change resource assignments and it will not impact the baseline data captured at the task level. Baselines can be updated by selecting specific tasks and then performing the Set Baseline function with the Selected tasks option. Note that Project will NOT re-calculate WBS level baseline totals/sub-totals unless the To summary tasks option is selected.
Second, from a standardized resource perspective, Project provides a very nice function to replace an existing resource already defined in your project pool with another resource. You can do this with the Add Resources > Build team from Enterprise function. Once you select an enterprise resource in the Build team window, you can select a resource in the current project resource pool and click the Replace button. All remaining work on the replaced resource is transferred to the newly added resource.
Additionally, this same function is also available in the Assign Resource window by selecting a resource in the Assign Resource window and then clicking the Replace button.
Finally, if the resource name already defined in the project pool exactly matches an enterprise resource name, when you save the schedule to the server, Project will prompt you to see if you want to change the resource currently defined in the resource pool to use the enterprise resource of the same name.
Hope that helps