If you follow that link to amazon I think you'll see why KQ probably changed it--they have it wrong there.
Actually are you sure the book company doesn't have it wrong? The Library of Congress has it "judgment" too: http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v1=1&ti=1,1&FT=james+blish+day+after&PID=15017&CNT=25+records+per+screen&SEQ=20011014130051&SID=4
Searching http://www.loc.gov/cgi-bin/formprocessor/copyright/locis.pl shows that it was registered first in serialized form as "The day after judgment":
2. Registration Number: RE-803-122 Title: [Contributions by James Blish] By aJames Blish. Note: Serialized novel. In: Galaxy magazine Claimant: Judith L. Blish (W) Effective Registration Date: 14Dec98 Original Class: BCONT: Issue Aug-Sept70. The day after judgment. PUB 23Jul70; B604348. Dec70. Dark side crossing. PUB 10Nov70; B625506.
The registration as "The day after judgement" (with the extra "e" was later:
1. Registration Number: RE-796-907 Title: The day after judgement. By James Blish. Claimant: acJudith L. Blish (W) Effective Registration Date: 9Feb99 Original Registration Date: 15Jan71; Original Registration Number: A210961. Original Class: A Claim Limit: NEW MATTER: additional text.
But oh well. So both are coomon. It's not an issue unless you want an entry on it, then a #REDIRECT from one to the other should probably keep people happy.
I think its a matter of english/american spelling: the English (Penguin books, 1974) paperback that I have has it as judgement, as does the 1st edition of the Encyclopedia of Science Fiction (ed. Peter Nicholls & John Clute, both english). It's not worth the argument really, I'll just go and correct a detail i got wrong instead -- Malcolm Farmer