Renege Top MM for EB?

Honestly depends on the BB/EB. Which one is it ? Do you have alumni there -> Can you be placed in a top group?

renege...it's not the end of the world

It's a very competitive market, and BB is generally better positioned for recessions. Just tell the MM you very recently received another offer, and you wouldn't do this unless you had no other choice, you're so sorry and you know it puts them in a tough situation but it's just too good to turn down. Be polite and apologize, but don't specify what firm you're reneging for.

It's a little rude to quit over email (in my opinion) so email HR and say something has come up that you'd like to discuss with them as soon as possible. But those "MM" firms are very much up there; I think they're considered more elite boutiques than MM. Definitely think hard about it and DO NOT DISCUSS RENEGING with any professional at either firm. You don't want anyone finding out until you have everything locked down. If your BB firm hears you reneged, it is possible they'll rescind their offer. Ideally, they would not ever find out.

When it comes to timelines, there are two ways to do it. Sign your BB offer and then renege, and you have BB cinched. Less likely to rescind if you've already signed and you can say you've already signed if MM/EB tries to pressure you. It'll help you to stay firm during the discussion knowing that as bad as it is to be on a MM/EB shit list, a BB shitlisting is probably worse and you've committed to this.

On the other hand, you'll then have two offers signed at one time; a very precarious position. If your MM/EB just refuses to let you renege, and you haven't signed the BB offer, you're still ok and can reject the offer from BB by saying you've already signed another offer but would love to keep in touch for future opportunities. Email HR and tell them that you would have signed no questions if you hadn't already signed elsewhere. Once the MM/EB firm hears you want to renege, you'll have lower chances of return offer bc they'll assume you are recruiting for someone else.

Also, avoid the word "renege"; you are not reneging, you are rescinding your acceptance (same thing, just a better connotation).

Those “MMs” are wildly different

Would renege. It happens more often than you think

Can't agree that if firm you've already signed at hears you want to renege and "doesn't allow it," then things will be no worse for the wear at that firm. Many at that firm will subconsciously hold it against you and you will poison that relationship in many people's eyes. I don't know the "right" answer, only that I certainly wouldn't take reneging as lightly as some on this post are -- not saying that you will for sure piss off a lot of people, but it's certainly a possibility and, worst case scenario, you could lose both offers.