@ARTICLE{Abazajian03,
   year = 2003,
   volume = 203,
   volume = 203,
@ARTICLE{Alexander03,
   year = 2003,
@ARTICLE{Anderson03,
   year = 2003,
@INPROCEEDINGS{Antonucci03,
   year = 2003,
@ARTICLE{Baes03,
   year = 2003,
   year = 2003,
....

sed 's/[a-z][A-Z]*03/2003/' infile.txt > outfile.txt