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#! /bin/bash
while read line
do
if [[ $line =~ "AB" ]]
then
line=${line/$(date -d "-1 day" +%Y%m%d)/$(date +%Y%m%d)}
echo $line
else
echo $line
fi...

Awk Solution:-

#!/bin/bash

YESTR=$( date -d "-1 day" +%Y%m%d )
TODAY=$( date +%Y%m%d )

awk -v YS=$YESTR -v TD=$TODAY ' {
if(NR==1) {
print; next;
}
if(match($0,/AB/)>0) {
...

# If the first occurrence of A,B.. to be replaced then..
sed 's/[A-Z]/1/' inputfile

# Replace the A,B.. in the alpha-numeric column
sed 's/\([0-9][0-9]*\)\([A-Z]\)\([0-9][0-9]\)/\11\3/' inputfile

awk '{sub("[A-Z]","1",$2)}1' file