A -> Y
B -> V
C -> Q
...

ABCDEFGHIJ

sed -i 's/A/Y/;s/B/V/' file

sed 's/A/Y/' file > file1; sed 's/B/V/' file > file1

$ cat sub.txt
A -> Y
B -> V
C -> Q
D -> K
E -> L
F -> O
G -> P
H -> W
I -> X
J -> Z

while read from a to; do ... ; done < sub.txt

while read from a to; do
    tr "$from" "$to" <<<"$string" > changed."$from".txt
done < sub.txt

$ ls changed*
changed.A.txt  changed.D.txt  changed.G.txt  changed.J.txt
changed.B.txt  changed.E.txt  changed.H.txt
changed.C.txt  changed.F.txt  changed.I.txt

$ for f in changed.*; do echo "=== $f ==="; cat $f; done
=== changed.A.txt ===
YBCDEFGHIJ
=== changed.B.txt ===
AVCDEFGHIJ
=== changed.C.txt ===
ABQDEFGHIJ
=== changed.D.txt ===
ABCKEFGHIJ
=== changed.E.txt ===
ABCDLFGHIJ
=== changed.F.txt ===
ABCDEOGHIJ
=== changed.G.txt ===
ABCDEFPHIJ
=== changed.H.txt ===
ABCDEFGWIJ
=== changed.I.txt ===
ABCDEFGHXJ
=== changed.J.txt ===
ABCDEFGHIZ

tmpFile=$(mktemp)
echo "$string" > "$tmpFile"
while read from a to; do
    tr "$from" "$to" < "$tmpFile" > changed."$from".txt
    cp changed."$from".txt "$tmpFile"
done < sub.txt

$ for f in changed.*; do echo "=== $f ==="; cat $f; done
=== changed.A.txt ===
YBCDEFGHIJ
=== changed.B.txt ===
YVCDEFGHIJ
=== changed.C.txt ===
YVQDEFGHIJ
=== changed.D.txt ===
YVQKEFGHIJ
=== changed.E.txt ===
YVQKLFGHIJ
=== changed.F.txt ===
YVQKLOGHIJ
=== changed.G.txt ===
YVQKLOPHIJ
=== changed.H.txt ===
YVQKLOPWIJ
=== changed.I.txt ===
YVQKLOPWXJ
=== changed.J.txt ===
YVQKLOPWXZ