It's difficult for me to know how bad those grades are, but looks like they were high enought to get an internship at a Tier-2. Usually criteria between MBB and Tier 2 is not very different, it's just that MBB has “first grabs” and Tier 2 open up their recruiting poll to more target schools in order to grab top talent.

Regarding internal transfer at a Big4, it can happen, but it is not easy. Doesn't happen very often. What you can try to do is not network to turn the offer you have into an offer from the service line you want there. You will have to network and be upfront with them that you won't accept that service line but you did enjoy the company. 

Bad grades?

• It will definitely be a negative impact factor
• However you can make-up with good targeted internships that you have done so far
• Best to check with HR/recruiting if they have any hard thresholds and if you are at all required to provide your grade information
• That said, your profile aside from your grades looks decent and if you network well, you might have a solid chance

Big4 internal transfer

• Instead of internal transfer a better route would be to apply as an experienced hire to other firms after a couple of years (and possibly a promotion as well)

Hi!

It's very hard to tell. If you don't have exceptional academic records I'd recommend investing time in networking to find a referral. Working first in tier 2 and then applying to MBB can also be helpful.

Hope this helps.

Best,

Anto