Do you strictly recommend a clean install of 9.00alpha?
No, you don’t have to do clean install. You can install without uninstalling the previous version. However, if you want to turn on new features such as Brackets/Quotation marks auto-comple, and “Indentation after #include”, you will need to check those options. You might also want to reset keyboard shortcuts so you can use F11 and CTRL + SHIFT + V for new features.
For future alpha versions, you will need to uninstall any existing alpha version before installing latest alpha version. However, you can still keep the current settings. When you uninstall the previous version, you will be prompted to save the current settings. Selecting YES will allow you reuse the existing settings when you install newer versions.