Hi

Kindly find my response to your inquiries below:
1.) how can I set easydiscuss up like easyblog that it shows : Only registered are allowed to read post? I changed the user
rights in each category and it shows if a guest try to write a new theme that he doesn't have the right to write a new theme.
But I want to show up the same page like in easyblog with the login page.
-> Unfortunately, there is no setting to shows the login page as the guest user cannot view the post/categories, it will not shows/totally hidden to them

2.) Can I use the login module from easyblog in easydiscuss?
-> You mean you want to put login module from easyblog in the easyblog page? Yes, you can, you only have to know which position in the page and menu that you want to put the module

3.) If I update easydiscuss will be all my handmade changes be deleted? How can I update without losing my changes?
-> If you have make changes in core file, it will be overrided as you undergo update process
If you have make changes in core file, better to backup all the file before updating
If the file can do template override, it should be great, because any file that in the template will not overrides
How to do template overrides? refer here: https://stackideas.com/docs/easydiscuss/administrators/templating/template-overrides