See after Diploma if you wish to get into BTech courses then you need to attempt lateral entry examination which provides you admission directly to the 2nd year of BE/BTech course. Now yeah! you can change your trade in your BTech/BE course and there will be no such restriction for the selection of the course. So yeah! you can go for BTech ECE after your Diploma in Mechanical Engineering.

To know about Mechanical Engineering visit:

https://www.careers360.com/courses/mechanical-engineering-course

To know about ECE visit:

https://www.careers360.com/courses/b-tech-in-electronics-and-communication-engineering

I hope this helps.