MCA

You can pursue M.Sc IT and M.Sc computer science. There are so many courses to pursue after BCA but the whole thing depends upon your interest. So, choose wisely.

B.Tech is an under grad. programme and is much favored by industries nowadays. MCA will be core computers and if you have a keen interest in the field of computers than MCA is a better option.

Please check out the college list below:
http://www.pune.ws/in/?list=colleges_institutes-mca-pune_university
And, college change or university change after the first year is completely dependent on college and university norms, some university allow it and some won't plus you need to have good scores in the first year to apply for the same. For further details contact the college authorities.

National Institute Of Technology- [NIT], Tiruchirappalli,
NIT, Warangal
and
JNU-Delhi are the top most colleges.

MCA is a sufficient qualification to start career as an IT professional (Developer or Tester). However, if you want to study more, you can do M.Tech CS by appearing for GATE. If you do not want to study further and not getting a job, then you may do short-term IT courses to obtain industry recognised certifications.

Both are good. It depends on your ability and college.

As you want to make career in the field of computer science, you are advised to prefer BCA-MCA over BSc-MCA. After BCA, you may take lateral entry admission in 2 year directly of MCA degree.