Bo,
Thanks for the reminder - somehow I had overlooked your request. I'll post it here for others to find in the future. I've developed this procedure from web site forums for several marques over the past 10 years or so, along with personal discussions with a friend who has a classic car upholstering company. Primary forum was a Ferrari one.
1. Clean the leather thoroughly using a good detergent and wet cloth to get all the old dirt etc. off the leather surface. Do this inside or at least in the shade, and let the leather dry slowly and out of the sun if it gets wet.
2. The critical (and some would say 'scary' step) is to clean all the old 'paint' off the leather surface that is no longer well bonded to the leather or to itself (modern leathers have acrylic paint on the surface rather than dye into the leather). Here you use a cloth (or the blue or white 'Shop Rags' that are on a roll) that is wetted with lacquer thinner (use good hand protection) and rub the surface of the leather. DO THIS OUTSIDE and in the shade!! Continue rubbing with new fresh lacquer thinner and clean 'rag' until VERY LITTLE or no more of the color comes off the leather onto the rag. You now have a VERY clean surface that will be receptive to further treatment.
3. I then use a leather softener (Leatherique, Soffener, Leather Therapy, etc.) to treat the leather and get it as soft as possible (assuming it is not new leather, and is probably somewhat dried out with age). I re-treat it with this softener every 24 hours or so until it seems that no more softener is being absorbed into the leather over another 24 hour period (may take 3-6 treatments).
4. As a final preparation for changing the color (or even refurbishing the same color), I now 'clean the surface' again once or twice with lacquer thinner to remove any of the softener that is on the surface, and prepare the remaining dye with a 'bite' to accept and bond with the soon to be applied leather dye (paint).
5. I have already purchased a good quality leather dye (I usually use dye from Color Plus - who also sells 'Soffener', but recently used dye from Leather Magic. Leather Magic has a LOT of OE dye formulas, so you don't have to pay extra for what other suppliers would term a 'custom color).
I apply the dye much as I do stain to a piece of wood. I use a clean piece of absorbent fabric and rub on a thin coat. After 48 hours or so, I apply another coat. I keep this up until I get a fully uniform color, and in the case of the grey leather in my car referenced above, I changed it to Mercedes Bamboo, and it took about 4- 6 thin applications to get it completely uniform. I then apply one or two additional coats allowing 3-4 days between coats.
6. I allow the leather dye to cure at least a week and usually 2-3 weeks before I put it into use.
This procedure has given me very durable and supple leather on Porsche, Mercedes, and now BMW leather seats/panels. It is quite labor intensive as you can see, but when completed, you have the satisfaction of leather that looks like new and is soft and durable. Also, IF you ever scratch/damage an area, you will have saved some of the 'dye', and can easily touch up the damaged area.
I ordered 2 qts of the MB Bamboo, and that was more than enough to treat all the side panels and the seating surfaces in my 3.0 CS.
Hope this is helpful.
