I just wanted to play along and wanted to submit a short function that I wrote in the past but had to find again... I know you have a solution but this one will work with whatever base length you want...

The lcChrs string is the basis. First char allows for blanks (in case left side has preceeding spaces. Then the last char in the list is the one that will trigger rollover to process next character....

You could even try calling with a sample loop and try with strings like

lcChrs = " 0123456789ABCDEF"
lcChrs = " ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789!@#$%^&*(){}"

as long as there are no duplicate characters in the string but the first space is needed as starting point for possible blank values...

function NextID
   lparameters lcID
   local lcChrs, lcNewID, lnLen, ln1, lnAt
   lcChrs	= " ABCDEFGHIJKLMNOPQRSTUVWXYZ"
   lcNewID	= ""
   lnLen	= len( lcID )
   for ln1 = lnLen to 1 step -1
      lnAt = at( substr( lcID, ln1, 1 ), lcChrs )
      if lnAt < len( lcChrs )
         */ No more carrying needed... get remaining chars
         */ not processed and add to chars already processed
         lcNewID = left( lcID, lnLen - (lnLen -ln1) -1 );
                 + substr( lcChrs, lnAt +1, 1 ) + lcNewID
         exit
      else
         lcNewID = "A" + lcNewID
      endif
   endfor
return( lcNewID )