Oh well according to the dev team this is "As Designed".
Someone at ESRI explain how are we to maintain globalids if we want to dump our existing data into a new empty schema if we cant change the globalID field to type guid under the append tool??
"When "use the field map to reconcile field differences" is selected in the Append tool for ArcGIS Pro 3.0, users are unable to modify the existing schema of the target dataset in the Field Map control. This is as-designed. Prior to ArcGIS Pro 3.0, the Field Map control gave the incorrect impression that fields could be added/removed and modified in the Append tool, but when the tool was actually run, these changes would not be reflected in the updated target dataset. This is because the schema of the target dataset is locked. Users are still able to specify input sources to map to target fields (i.e. reconcile field differences) that are present in the existing schema. In ArcMap the controls to add/remove fields in the Field Map for the Append tool are disabled for this same reason. The change to the Pro 3.0 Append Field Map control was done to give an accurate representation of what could be accomplished in the tool. Documentation for Pro 3.0 states that "Because the data of the input datasets is written to an existing target dataset that has a predefined schema (field definitions), the Field Map parameter does not allow fields to be added or removed from the target dataset" (https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/append.htm). This has not changed from earlier versions of Pro. For example refer to archived 2.6 documentation at https://pro.arcgis.com/en/pro-app/2.6/tool-reference/data-management/append.htm. We will further review the documentation to ensure all tool capabilities are properly communicated. The Merge tool is recommended for combining multiple input datasets with the ability to modify existing fields.