What makes it hard to do in a regexp is that "JAZJ" is not "JAZZ", yet you don't want to replace it. This is not the job of a regexp.

Two ways:

• s/
     .
     (?<!J(?=AZZ))
     (?<!JA(?=ZZ))
     (?<!JAZ(?=Z))
     (?<!JAZZ)
  /a/xgs;
  [download]

• s/~/a/g;
  s/JAZZ/~/g;
  s/[^~]/a/g;
  s/~/JAZZ/g;
  [download]

• s/(JAZZ|.)/$1 eq 'JAZZ' ? $1 : 'a'/egs;
  [download]

• s/(JAZZ)|./defined($1) ? $1 : 'a'/egs;
  [download]

• s/((?:JAZZ)*)./$1a/gs;
  [download]

Update: Or more.

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