PLEASE HELP! 18'' 20'' 21'' TIRERS! SENSORS...

[BrazilianE70]

TIRES:

I'd figure I ask the experts here on the forum so I don't mess my SAV up for lack of knowledge. I have a 2008 X5 3.0si that came stock with Style 210 18" wheels and Bridgestone Dueler HL 255/55r18 tires all around. This past summer I came across a good deal on a set of 21" Style 215 wheels with no tires. Searched online and found out that from factory they come with Dunlop Sportmaxx SP "325/30/R21 REAR" and "285/35/R21 FRONT". So I purchased the tires and mounted them and all fitted and drove nicely. But a friend raised a concern when he mentioned that the overall diameter should be same all around due to the car being AWD or I'd be looking at a damaged differential. So is tire size setup I have on the 21"s incorrect? And I recently got a set of 20" Y spoke style 214 wheels to replace the 18"s on the winter. Tire setup again is different rears from the front. 315/35R20 REAR and 275/40R20 FRONT. So is it ok to run these configurations on my X5, or will it cause any damage? Idea is to have the big 21"s during summer and the nice 20"s during winter.


TPM SENSORS:

As far as TPM sensors, another whole dilemma I'm having. The 21"s came with some BMW TPM sensor that the guy I bought the wheels from said they would work fine with my X5. Fact is the tire pressure yellow light and warning popped on the dash and wouldn't reset. My mechanic tried to configure it with his SnapOn tool with no luck. Is this something that only the BMW dealer can program/configure? Soon as I swapped back to the stock 18"s the car recognized and reset the light and was all good. Should I swap out the TPM sensors from my stock 18"s to the 20"s & 21"s when I swap wheels?