Can I change my university from GTU to Bangalore University after my 1st semester of MBA?

That you can't change. As you are pursuing MBA, then complete it from one institution, as you may have a serious consequences during your placement process. So, always choose a good university for MBA as ROI should be optimal.

I guess, you can change college in the same university not the university.

Yes, in first year you have a flexibility to change the college, but after that you can't. So, make your decisions accordingly. All the best for your bright future.

Yes, you can. It is allowed.

You can surely change, if the university provides lateral entry as an option.

Changing the university is pretty much possible for the same course, however you will need to check if there are lateral entry options available in Bangalore University whichever you wish to apply for.