- Will the same GB file work in Logic Pro or do I need to start with the beginning with all arrangements from scratch?
You don't have to start from scratch. You can import your Garage Band files into Logic Pro and they should open up pretty much exactly how they were in Garage Band. Note: You can't go back. Once you make changes in Logic, you won't be able to easily open that file up in Garage Band.
- If I am able to move to Logic Pro from GB - is there is any possibility of degrade of sound quality?
No, more likely the opposite. Logic Pro is Apple's top of the line audio editing software. It has all of their best features and sound.
- What will be the key benefits I can get by migrating from GB to Logic
You will have a lot more power for recording, editing and mixing. You will have access to many more effect plugins and virtual instruments. You'll have extended professional capabilities like alternate tunings, clock synchronization, etc.
- Is Logic Pro as user friendly as GB?
Sort of, but not really. Your first day in Logic Pro will probably not be too hard. There are a lot of similarities with Garage Band. Once you start looking at using the more advanced features that Logic Pro has that are not in Garage Band, you'll find there is a learning curve. Some people find the menu locations and layout of features in Logic to be very intuitive, others do not. There isn't really a way to find out without using the software.
The Apple documentation that I've read on Logic Pro is usually pretty brief and doesn't do a good job of explaining all the details of each function and feature. I find that very disappointing for "professional" software. At the same time, there are many great videos on YouTube about how to use advanced features in Logic Pro.
On a personal note, once I got Logic Pro, I never wanted to go back to Garage Band. It is definitely an upgrade.