Don't bother, I don't understand swapping model for model just due to miles.
It seems to me to be a pointless idea and ultimately a waste of money.
No guarantee the "new" car will be any better than what you have now.
The only guarantee seems to be a payout in doing the swap, I'd rather bank it if in the unlikely event the current car drops a big bill which is no more likely to happen than the new one doing so if you follow......
I remember looking at a late 100k 528i when I had my 260k+ example I could have bought it for £2995, in other words £2200 ish over what mine cost me to drive the same thing with 160k less miles, it was quite reassuring that the 100k miler actually felt no better than my own example. I wasn't seriously considering the swap, just comparing like with like, I certainly wouldn't have felt I'd gained anything other than a hole in my pocket had I actually swapped cars.