If you qualify AP ECET with decent rank then you can easily shift your field. There are no such restrictions regarding subject combination. So don't worry about it just prepare well and you can get admission in a college that accept the AP ECET course.

To know about the syllabus of AP ECET for ECE copy paste the link below in your browser:

https://sche.ap.gov.in/ECET/PDF/Diploma%20Holders%20in%20Engineering/ELECTRONICS%20AND%20INSTRUMENTATION%20ENGINEERING.pdf

I hope my answer helps. All the very best for your future endeavors!