Formula to determine which letter is shown in a cell

Good morning,
I've stumbled across a hurdle I don't know how to clear.
I'm using the following formula to output a lot code into a cell.

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If you'll take notice, the letter "B" is right smack-dab in the middle. I've come across a scenario that I might need it to substitute an "S" for the "B".
Example output:
C17MAR2015 B107 --- this is how 99% of our documents need to look. However, we are quickly going to need this document to have the code like this:
C17MAR2015 S107 --- is it possible to use a check-box form control to actively select the B or S?

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I've been using this line of code and a check box to make the text "Rush Testing Requested" show up or not. Can that type of code me integrated into my lot code formula?

something like: ="C"&UPPER(TEXT(I9,"DDMMMYYYY"))&" "&"IF('Batch Review Checklist'!J11=TRUE,"S","")"&'Print Control'!D8 -- I know this won't work as is because I'm sure how to create the argument where, by default, false would = "B".
Any thoughts?
Thanks!