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...
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS