Interesting. I downgraded my BahnCard First 25 last week. If anyone else is checking in here and needs to know, you can't actually simply downgrade. (Upgrading the BahnCard is different matter and very easy).

You have to officially cancel the BahnCard First, and fill out the application form for a new normal (2nd class) BahnCard 25. The fact that you cancel (kundigen) does not mean that you lose the points, because the points are attached to your name, not the BahnCard. So two different service center people told me.

You can still get a 1.5 points ratio for buying 1st Class tickets if you pay using the the BahnCard credit card (you can get 1,500 bonus points for taking out the BahnCard credit card).

My question of the day: is it still ok to use the 1st class upgrades if one only has the normal BahnCard, not the BahnCard 1st?