Reneging mid BB for another mid BB??

Would not recommend

I think people would not care that much about you reneging but given how small the industry is, I would avoid burning bridges and just stay put. After 6 months on the job, you can start looking for a lateral move

Only 6 months?? I was told a year at least. Won’t firms think Im a flight risk? I already switched banks from SA to FT too

Honestly, it's up to you. Realize that you will be burning some bridges in choosing to renege though. You'll probably be blacklisted from the BB you are leaving and you might be remembered by some of the staff for your first years.

However, note that in reality, you are easily replaceable. People are hired and fired from banks all the time. They don't really care. You're just another one of the many who got away. They'll never think of you again if you leave. The only place you'll be noted, is their black list. 

For most, this is a worthy trade off. Just be aware of the possible consiquences

From what I know Barclays is a generalist offer for most groups so you won’t be guaranteed group placement. It’s hard to make the switch for that reason but generally you should just renege and make minimal information known about the switch.

How not so good is the group at CS? If it’s a group that you have absolutely no interest in (a securitization type group or something along those lines), then I’d say go for it. If its a middle of the pack coverage group at CS, then I’d say the risk may not be worth it, since as others mentioned barclays has generalist full-time offers and placement can be tough.

Know people who have reneged on MM for BBs and it worked out for them, but there is always a risk

I wouldn't. I know an MD who once did something super petty to a kid he found annoying. It's not a big change, so I wouldn't risk pissing anyone off (even if it's unwarrented)