If you seek admission in BTech after diploma then you have to go for Lateral Entry examination. Now if you qualify any lateral entry then in general you can change your subject to any other department such as Civil, ELectronics, ELectrical etc. But most of the college offering BTech Agricultural Engineering via lateral entry examination asks for an Diploma in Agricultural engineering only. Thus depending on which college you are taking admission in, its entirely dependent whether you are eligible or not.

For BTech Lateral Entry examination list and eligibilities you may visit the link below:

https://engineering.careers360.com/articles/what-btech-lateral-entry-eligibility

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