I'd say ... Sell the 400D. The lenses, you can wait a bit and see if you need them on the 7D.
BTW, there is no such thing as a loss selling gear you never use. You can always use the proceeds to get stuff you really need.
Actually, I wouldn't much miss the 18-55. The 55-250 I'm not so sure about though - it is very compact and the image quality isn't bad at all. The 18-55 I could sell without the filter and put the latter on my 70-210.
The thing is I kinda like the 400D. Looking at the back it's like a mini 5D MkII with the buttons on the left, one of the reasons I bought it second-hand at the time. Emotions get in the way of reason...
5Dmk3, 5Dmk2, Sony NEX-6 | SY14mm f/2.8, Ʃ20mm f/1.8, 35mm f/2, Ʃ35mm f/1.4A, 50mm f/1.8 I, Ʃ50mm f/1.4 EX, 100mm f/2.8L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: 16-50 OSS, Ʃ30mm f/2.8 EX DN, 55-210 OSS | 2x FT-QL, AE-1 Program, some FD(n) & FL primes