First, yes, we take great care to ensure that every new version of MultiChain is backwards compatible with previous protocol versions.
Second, protocol 10008 includes preparation for a blockchain upgrading mechanism, which will allow the parameters of a blockchain, including the protocol version, to be upgraded in place. The new set of parameters will affect all of the blocks in the chain after a certain point. You can't try this yet, but this is where we're going in future, so protocol 10008 is effectively future-proofed.
