It will depend on how they amend the rules but I’d expect that interest will be adjusted on all loans, old and new, from the point they update the rules, not retrospective. City will likely complain about that but if they moan too hard then they will lose one of their allies as Everton will be shafted by it, massively so.
Arsenal and others can simply convert their loans into shares so that they don’t have to charge interest going forward. I think that’s one of the reasons the PL were relaxed about it in the first place. It’s really all just paper trail. Theoretically, it’s harder to pull your money out if it’s in the form of shares rather than a loan, but I think most shareholders know that when they make a loan, they’re not getting that money back out anyway. Only exceptions I’m aware of are Brighton who repaid some of their debt after the Caicedo sale, and Man U, who pay a high rate of interest, but who haven’t repaid any of the Glazers’ debt balance to my knowledge.
