Code:
$ awk -F, '/INSERT/{for(i=1;i<=NF;i++){if ($i ~ /lastModified/){x=i;print;getline;break;}}}{y="0000-0000 00:00:00";sub(/[-0-9: ]+/,y,$x);}1' OFS=, file
INSERT INTO `table1` (`id`, `action`, `date`, `descrip`, `lastModified`) VALUES
(1,'Change','2011-05-05 00:00:00','Account Updated','0000-0000 00:00:00'),
(2,'dsfsd','2000-04-06 00:00:00','Updated','0000-0000 00:00:00'),
(3,'Change','2011-05-05 00:00:00','Account Updated','0000-0000 00:00:00');
INSERT INTO `table2` (`id`, `action`, `date`, `lastModified`, `dynamics`) VALUES
(1,'Change','2011-05-05 00:00:00','0000-0000 00:00:00',null),
(2,'dsfsd','2000-04-06 00:00:00','0000-0000 00:00:00',null),
(3,'Change','2011-05-05 00:00:00','0000-0000 00:00:00',123);