Thanks for all the advice everyone, tried it today, brakes rubbed so had to clean pistons and push them back in as they had auto adjusted to the width of the older discs, the gears 'work' but certainly need some adjusting. I was hoping the set ups would interchange seamlessly as I'm a bit of a lzzy git but I think the lesser of the evils/ least amount of faff is to use the one set of discs and cassette on both wheelsets. Thanks again for all the great advice.