Correct it is a freaking huge change of monumental proportions.
Its a high-risk high-reward gambit that expands the potential in all sorts of currently unforeseen ways..
It really can't be implemented without the change,
Well... some special cases things can be done, (with the federation/oracles) but the vast capabilities of validating arbitrary code on (and with) the Bitcoin block chain is what takes the fork.
It'd be interesting if you could expand on that.
My understanding is any sidechain scheme can be implemented by replacing SPVproof with federation/oracles. It was even suggested by the developers that this would be the first step of the implementation for most sidechains (bootstrapping on a federated peg model) then implemented through OP_SPV.
Is this not the case?