You have to rebuild your output string. I would suggest a series of while loops (extremely simple ones) that use $reptok rather than $replace since you obviously don't want "qwertyuiop" to be replaced by "qwerThank Youuiop".
Code:

  var %a = $1-
  while ($istok(%a, ty, 32))  %a = $reptok(%a,  ty, $+($wc1(T),hank $wc1(Y),ou)               , 1, 32)
  while ($istok(%a, bbl, 32)) %a = $reptok(%a, bbl, $+($wc1(B),e    $wc1(B),ack  $wc1(L),ater), 1, 32)
  while ($istok(%a, brb, 32)) %a = $reptok(%a, brb, $+($wc1(B),e    $wc1(R),ight $wc1(B),ack) , 1, 32)
  while ($istok(%a, tyt, 32)) %a = $reptok(%a, tyt, $+($wc1(T),ake  $wc1(Y),our  $wc1(T),ime) , 1, 32)
  say %a
  halt
}

Another way to do the exact same job (without loops) is to use $regsub:
Code:

  var %a = $1-
  !.echo -q $regsub(%a, / \b (ty ) ([^\w]) /gix, $+($wc1(T),hank $wc1(Y),ou)\2               , %a)
  !.echo -q $regsub(%a, / \b (bbl) ([^\w]) /gix, $+($wc1(B),e    $wc1(B),ack  $wc1(L),ater)\2, %a)
  !.echo -q $regsub(%a, / \b (brb) ([^\w]) /gix, $+($wc1(B),e    $wc1(R),ight $wc1(B),ack)\2 , %a)
  !.echo -q $regsub(%a, / \b (tyt) ([^\w]) /gix, $+($wc1(T),ake  $wc1(Y),our  $wc1(T),ime)\2 , %a)
  say %a
  halt
}

cool


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