Electronics and Communication Engineering

It can not be done. You will not be offered admission into IIT through lateral scheme. The reason being that Indian Institute of Technology does not offer lateral entry to Engineering aspirants. It is not possible in the IITs.
It is not just the lateral entry, you will not get any sort of admission for Engineering in the IIT. To be eligible for B.Tech from the IITs you need to appear for entrance exam called the IIT-JEE which is gonna be replaced by the ISEET next year.
In order to appear for these entrance exams you must have passed 12th with PCM as the subjects. Since diploma course is taken after the 10th standard, it is also not possible for you to do so.
Therefore you should forget about pursuing B.Tech from IITs. There are number of other good universities which would offer you lateral entry, unfortunately IIT is not one of them.
Lateral entry scheme is quite popular in India. So much so that most of the universities reserve around 10 percent of the total seats for the lateral entry candidates. The admission is given on either merit or through entrance exam.
Now for admission through entrance exam you are not suppose to appear for AIEEE, IIT-JEE etc. Like I mentioned earlier, these exams are for candidates who have completed 12th. The entrance exam for you would be LEET. There are various other exams as well, some universities like to conduct their own entrance exam.
It is also possible to seek admission on merit. Various universities offer direct admission to the diploma candidates. However they require you to complete diploma course with at least 60 percent marks to do so.

If you do B.Tech ECE, you have following major job opportunities:
(a) Operations and Maintenance Engineering roles in Service and Manufacturing sector.
(b) Designer in Electronics and Telecom companies
(c) Network Engineering roles.
(d) Work in Electronics and Telecom equipment manufacturers and its ancillary industries in various roles.
(e) R&D org such as DRDO, ISRO, etc.
(f) Indian Engineering Services
(g) Defence and Paramilitary forces
(h) Merchant Navy and Airlines in maintenance roles
(I) Education field
(j) IT and ITES companies
(k) Entrepreneurship
(m) All Government , PSU and Public Sector Banks where eligibility criteria is graduation degree etc
Career opportunities for ECE grads is endless.

An eight week long Summer Research Internship Program (SRIP) within the summer vacation period of the institute. The main aim is to provide an opportunity and exposure to the outstanding senior level that is 3rd or pre-final year undergraduate students (including B.Tech-M.Tech dual degree students) to interact with the faculty members and state of the art research facilities of the institute.
Who can apply: Interested students from IITs, NITs, IIITs and other reputed central/state institutions from all over the country can apply. The offer will be selective.
Number of Internships: 20.
Financials: All the selected interns (paid/unpaid) are required to pay to the institute fee of Rs. 10,000/- for the lodging, boarding and other admissible charges during their stay at IIIT Allahabad. The interns will be provided with a suitable hostel accommodation on campus. Those who are selected for the stipend will be given Rs.10,000 separately by the institute at the end of the completion of the training.
This is goos as the main aim is to provide an opportunity and exposure to the outstanding senior level (3rd Year/pre-final year) undergraduate students (including B.Tech-M.Tech Dual Degree) are also encouraged to apply to interact with the world class faculty members and state of the art research facilities of the institute.

Physical cosmology is a branch of physics and astrophysics that deals with the study of the physical origins and evolution of the universe. It also includes the study of the nature of the universe on a large scale.
I think the most important thing one needs in a profession like astronomy and cosmology is curiosity. The conventional way is to go for Bachelors in physics, followed by a Masters and a PhD in physics. But some people are doing PhD after a Bachelors in engineering. One needs very good understanding of physics and maths as a prerequisite. Meanwhile, many leading research institutes like IUCAA, Pune organise short term Schools in astrophysics for school students, which should be used to get a deeper glimpse.