Apart from prior chrtran() and strtran() offerings... you might avoid the duplicate substr():

lcString="ABC,DEF,GHI,JKL"
lcString2=""
x=0
do while x<len(m.lcString)
  x=m.x+1
lcString3=SUBSTR(m.lcString,m.x,1)
  IF m.lcString3<>","
    lcString2=m.lcString2+m.lcString3
  ENDIF
enddo X
?m.lcString2

For enjoyers of left field:

lcString="ABC,DEF,GHI,JKL"
lcString2=""
licount=getwordcount(m.lcString,",")
for x=1 to m.liCount
   lcString2=m.lcString2+getwordnum(m.lcString,m.x,",")
endfor
?m.lcString2

And then there's always strextract()...