Using Loop function with arcpy.Exists()?

I have a code snippet using elif that checks if a file exists and continues to check through the letters of the alphabet (for example _A, _B, _C etc). All works fine but I think there must a better (or cleaner) way to write the code using a loop function instead of the way I have done.

#Create MXD file path
mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01.mxd"
mxdPath = wrFolder + "/M_" + WorkrequestRaw +"_01"

#check for latest version and create next letter
if  not arcpy.Exists(mxdOutput):
    mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01.mxd"
    pdfversion = ""
elif not arcpy.Exists(mxdPath +"_A.mxd"):
    mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_A.mxd"
    pdfversion = "A"
elif not arcpy.Exists(mxdPath +"_B.mxd"):
    mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_B.mxd"
    pdfversion = "B"
elif not arcpy.Exists(mxdPath +"_C.mxd"):
    mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_C.mxd"
    pdfversion = "C"
elif not arcpy.Exists(mxdPath +"_D.mxd"):
    mxdOutput = wrFolder + "/M_" + WorkrequestRaw +"_01_D.mxd"
    pdfversion = "D"

I am a self taught beginner and trying to keep the code tidy. Welcome any comments on a better methodology?

Update poorly written question: What I am trying to do is if file name exists write new file called ‘filenameA’ and if ‘filenameA’ exists write ‘filenameB’ and if ‘filenameB exists write ‘filenameC’ etc.