I have a set of Michelin A/S 3plus tires on my E55, stock sizes. The front are in good shape but the back are getting to the point of replacement. The tires have ~17000 miles. Michelin no longer makes the tire so I guess I could replace the rears with Michelin A/S 4's but then I'm mixing tires by axle. OK to mix?

I also have some larger SL550 rims I could "up size" all 4 tires and call it a day.

Stock E55
245/40/18 Front
265/35/18 Rear

Versus

SL550 Rims
255/40/18 Front, (it seems you need a 3mm spacer)
285/35/18 Rear

Rambling a bit...

 

Mixing AS3 and AS4 is probably alright. People have mixed far, far worse.

 

But when you have the opportunity to correct this, I would.