Help can't restore

[1xsamuraix1]

My iphone is already jailbroken with ultrasnow and I wanna switch to greenpoison but when I put my phone in dfu mode and try to restore to 4.2.1 firmware it give me this message:

iPhone "iPhone" could not be restored. the device isn't eligible for the requested build.

I don't know what do help?

 

[JxM]

Do you have your SHSH blobs backed up for 4.2.1? On your computer or through Cydia?