Gorkha G,
I think it's the best time for you to switch if you want to.
If you want to migrate to Canada, then petrolium engg. is the right one to choose. But as you are concerned in Bio-Diesel stuff esp. in Nepal, then I should say that's not a good choice. You, me and all sajha users know how Nepal is and how the projects run there. For bio-diesel you first need infrastructure for it, that's why it's way too expensive than hydro in our country.
Mechanical is not too bad.
But, finally, it's your interest and passion towards it. The place where you have interest ......you can concentrate more towards that interest.....and I hope it would become your passion. However, for sure, that energy should be poured looking at its future prospects and commercial aspects. Both mechanical and petrolium look great but it eventually, as I mentioned, it all depends upon your interest.
Hope that helped you a bit.
-daum