Formula To Replace Values If Another Exists?

So I have this formula to basically lookup the letters in column L and if certain letters exist, then change those letters to another letter. Here is my Formula:

=IF(L1="A",IF(ISNUMBER(MATCH("B",$L$1:$L$200,0)),"B",L1),IF(L1="D",IF(ISNUMBER(MATCH("E",$L$1:$L$200,0)),"E",L1),
IF(L1="L",IF(ISNUMBER(MATCH("L",$L$1:$L$200,0)),"O",L2),IF(L2="F",IF(ISNUMBER(MATCH("K",$L$1:$L$200,0)),"K",L1),L1))))

For example: if column L looks like

A
B
A
E
D

Then it would convert the A's to B's since there is a B present and D's to E's, but if the B and E hadn't been in the equation then it would leave the A's and D's as they are. WIth this formula the output looks like:
B
B
B
E
E

My problem with this formula is that it only recognizes the first letter in each column. Sometimes I have multiple letters in a column and I need it to recognize them and change accordingly.

For example:
A
D_B
E

I would like to have it change to
B
E_B
E

I have attached a sample workbook that may make it more clear... If a VBA code would work better then I would be open to that as well. Combining question.xlsx