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

If anyone could help me with this problem I would be very appreciative!

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

That almost works but i don't want to completely replace the letter with the other, only if the other letter is also present in the array. So if "A" is present and "B" is not then keep the "A" an "A". However, only if a "B" is present then change the "A" or "A"s to "B" or "B"s. THank you for your reply.

I'm referring to the contents of column A. In other words, if Column A contains an "A" keep it as an "A" but if it contains a "B" change all the "A"s to "B"s. The formula I have does this perfectly except I need it to detect multiple values in a cell and change those as well, as in my example from the first post.