As the title says, I've managed to find a role and have already accepted and signed everything. However, a recruiter from the company which I interned for previously just reached out and I would 100% jump ship and join them if the opportunity arises. I know that I shouldn't be quick to renege my current role before everything is set in stone, but I'm looking for advice on how to best navigate this.

What do I tell this recruiter? Should I be upfront or say that I'm still open to roles? The answer to that might be obvious, but what about salary negotiations? I might be getting ahead of myself, but if I'm presented with an offer, how do I negotiate a better salary without revealing that I've already signed an offer? This has never happened to me before so I'm generally just looking for advice. Anything helps, thanks in advance.