You do not need to upgrade. However, you can only work in the version that your modeler is at. So if you open an existing older project in a 7.23.4 modeler, the modeler will upgrade the project to 7.23.4. The same goes for starting a new project.
However, you are free to download the 7.23.3 modeler version and start a new blank app with 7.23.3
Actually, I tried it from the 7.23.3 modeler, and Karthigaiselvi Paulraj is right. When trying to start from Blank App, you are prompted to start from 7.23.4
However, when I tried version 7.13.1 it works fine.
It could be that Mendix fixed a certain issue in 7.23.3 and from now on don't want you to start a Blank App from this version.
No, you can start a new project in any available version of the modeler. However, the New App / Edit App button in the mendix.com environment often wants to start in the newest version of the modeler.
If you want to start a project in a specific version of the modeler, it's easiest to create that project from the modeler.