I actually have done the h8 swaps on my last e90... write up here:
http://www.e90post.com/forums/showthread.php?t=558077 - dead easy.
As for a how to guide on replacing E90 H7 dipped.... put your wheel on full lock and in the inner arch you'll see a plastic panel that sits behind your headlights, they have two large flat head style grommets at the top of the flap, I used some plastic trim to turn these 180 degrees, remove the flap. push to remove metal clip and remove the rear cover, remove the connector from the bulb, push the clip toward the headlight lens and down to release the h7 bulb and reverse to put it all back together.
The ones I bought were
http://www.ebay.co.uk/itm/2906484507...#ht_1549wt_906 took about a week for delivery