You may be better off just buying a complete new set of sensors. If I remember correctly they have batteries and will go dead after 6 or 8 years (or something like that).
That would be a major pain to have just installed the old sensors with new stems in your new rims and then have a sensor battery go dead, so you'd have to unmount the tire again just to replace the sensor.
It's easy to replace them all now and start from brand new all around. That's what I would do.
I think it's about $200 for a complete set of all 4 sensors and stems. Don't buy direct from BMW as they will be more expensive. I believe we have the 433MHz Beru sensors, which you can find online.
For example, I quickly searched and found this:
http://www.bavarianx.com/BMW_TPMS_Ti...wz4-r8-kit.htm