I've recently bought a Dymanique S and really like the alloys on them so plan on keeping them. The tyres currently have 50 profile on and I wish to fit 40s to them! The two front tyres are needing replacement now, whilst the rear tyres have a few months left on them. Would I be okay running 40 on the front and 50 on the back until the rears get changed or should they all be changed together?