Thanks a lot for all your replies.
I have the money to sell my 400D + 15-85 and buy a 6D with an used 24 - 105, but my wife will kill me
Before I baught the 70 - 200 F4 L IS USM, I said to me, 1000 - 1200€ for a new body will be ok.
But on the last bicycle trip around our local inland lake, I missed the focal length to take pictures of bird (goose, heron and so on) and found a cheap 70 - 200 (used - 640€) on ebay.
Actually i just need a better IQ at low light situations (over ISO 400) to take indoor pictures of my 3 weeks old daughter.
I want to have good pictures of my daughter, but I think the moment for an upgrade is not so good.
One possibility is, I buy an used 550D for crossing, look how it fits my needs, and upgrade when I really know in which direction (xxD,7D or full frame) I want to go
Or I say to me, that at the moment the IQ of the 400D on ISO 800 and 1600 is ok for this family photos (I'm no pro), save some money; wait until the end of the year and compare the prices of the different cameras (60D, 70D, 7D and 6D)
I'm affraid of making a wrong decision
But you can't decide for me, I know