Is there a script for Find and Replace assigned Keywords?

@Shlomit Heymann – so how did the script work for you?

Yes, there is. If you replace it with a new keyword, you will need to convert it to a persistent keyword if you wish for it to be part of your library. 

// https://stackoverflow.com/questions/47797219/edit-script-for-editing-metadata-in-adobe-bridge-how-should-i-put-key-words
#target bridge

   if( BridgeTalk.appName == "bridge" ) {
keyReplace = MenuElement.create("command", "Add-Replace-Remove Keyword", "at the end of tools");
}
keyReplace.onSelect = function () {
      mainReplaceKeyword();
      }

function mainReplaceKeyword(){
if(app.version.substr(0,app.version.indexOf('.'))==1){
alert("Sorry You Need CS3 or CS4 to run this script!");
return;
    }
var dlg =
"dialog{text:'Script Interface',bounds:[100,100,500,310],"+
"panel0:Panel{bounds:[10,10,390,200] , text:'' ,properties:{borderStyle:'etched',su1PanelCoordinates:true},"+
"title:StaticText{bounds:[60,10,350,40] , text:'Add/Replace/Remove Keyword' ,properties:{scrolling:undefined,multiline:undefined}},"+
"panel1:Panel{bounds:[10,40,370,150] , text:'' ,properties:{borderStyle:'etched',su1PanelCoordinates:true},"+
"addKey:Checkbox{bounds:[20,10,160,31] , text:'Add Keyword' },"+
"statictext1:StaticText{bounds:[20,40,119,60] , text:'Replace' ,properties:{scrolling:undefined,multiline:undefined}},"+
"From:EditText{bounds:[120,40,350,60] , text:'' ,properties:{multiline:false,noecho:false,readonly:false}},"+
"statictext2:StaticText{bounds:[20,80,90,97] , text:'With' ,properties:{scrolling:undefined,multiline:undefined}},"+
"To:EditText{bounds:[120,80,350,100] , text:'' ,properties:{multiline:false,noecho:false,readonly:false}}},"+
"button0:Button{bounds:[10,160,180,181] , text:'Ok' },"+
"button1:Button{bounds:[200,160,370,181] , text:'Cancel' }}};";
var win = new Window(dlg,"Replace Keyword");
win.center();

win.panel0.title.graphics.font = ScriptUI.newFont("Times","BOLDITALIC",20);
g = win.graphics;
b=win.panel0.title.graphics;
var myBrush = g.newBrush(g.BrushType.SOLID_COLOR, [0.99, 0.99, 0.20, 1]);
g.backgroundColor = myBrush;
var myPen =b.newPen (g.PenType.SOLID_COLOR, [0.00, 0.00, 0.99, 1],lineWidth=1);
var myPen2 =b.newPen (g.PenType.SOLID_COLOR, [0.99, 0.00, 0.00, 1],lineWidth=1);
g.foregroundColor = myPen;
b.foregroundColor = myPen2;
win.panel0.panel1.From.active=true;
win.panel0.panel1.addKey.onClick = function() {
if(win.panel0.panel1.addKey.value) {
    win.panel0.panel1.statictext1.text = "New Keyword";
    win.panel0.panel1.From.active=true;
    win.panel0.panel1.statictext2.visible=false;
    win.panel0.panel1.To.visible=false;
    }
if(!win.panel0.panel1.addKey.value) {
    win.panel0.panel1.statictext1.text = "Replace";
    win.panel0.panel1.statictext2.visible=true;
    win.panel0.panel1.To.visible=true;
    win.panel0.panel1.From.active=true;
    }
}
win.center();
var done = false;
    while (!done) {
      var x = win.show();
      if (x == 0 || x == 2) {
        win.canceled = true;
        done = true;
      } else if (x == 1) {
        done = true;
       var result = valiDate();
        if(result != true) {
            alert(result);
            return;
        }else{
            var Replace = win.panel0.panel1.From.text;
            var With = win.panel0.panel1.To.text;
            var addKey = win.panel0.panel1.addKey.value;
            processKeyword(Replace,With,addKey);
        }
      }
   }

function valiDate(){
    if(win.panel0.panel1.From.text =='') return "No Keyword Entered!";
return true;
}
function processKeyword(Replace,Witha,ddKey){
try{
loadXMPScript();
}catch(e){
alert("Can not load XMPScript\r" + e.message);
    }

var items = app.document.selections;
  for (var i = 0; i < items.length; i++){
      var file=new Thumbnail(items[i]);
try{
var xmpFile = new XMPFile(file.path, XMPConst.UNKNOWN,XMPConst.OPEN_FOR_UPDATE);
}catch(e){
        alert("Problem opening xmp for update:-\r" + file.path +"\r" +e.message);
        return;
    }
try{
var xmp = xmpFile.getXMP();
}catch(e){
        alert("Problem opening xmp data:-\r"  + e.message);
        return;
    }
try{
var tmpCount =  xmp.countArrayItems(XMPConst.NS_DC, "subject");
}catch(e){
    alert("Cannot get count \r" + e.message);
    }
            if(addKey){
                xmp.appendArrayItem(XMPConst.NS_DC, "subject", Replace, 0,XMPConst.ARRAY_IS_ORDERED);
                }
if(tmpCount >0 && !addKey){
        for (var a =0;a<tmpCount;a++){
            var     Keyword = xmp.getArrayItem(XMPConst.NS_DC,'subject', a+1);
            if(Keyword == Replace && With == '') {
                xmp.deleteArrayItem(XMPConst.NS_DC,'subject', a+1);
                }
            if(Keyword == Replace && With != ''){
                xmp.setArrayItem(XMPConst.NS_DC,'subject', a+1,With);
                }
        }
}

if (xmpFile.canPutXMP(xmp)) {
xmpFile.putXMP(xmp);
}else{
alert(e.message);
    }
xmpFile.closeFile(XMPConst.CLOSE_UPDATE_SAFELY);
}
unloadXMPScript();
}
}

function loadXMPScript()
{
    var results = new XMPLibMsg("XMPScript Library already loaded", 0, false);

    if (!ExternalObject.AdobeXMPScript)
    {
        try
        {
            ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
            results.message = "XMPScript Library loaded";
        }
        catch (e)
        {
            alert("Could not load AdobeXMPScript \r" + e.message);
            results.message = "ERROR Loading AdobeXMPScript: " + e;
            results.line = e.line;
            results.error = true;
        }
    }

    return results;
}
function unloadXMPScript()
{
    var results = new XMPLibMsg("XMPScript Library not loaded", 0, false);

    if( ExternalObject.AdobeXMPScript )
    {
        try
        {
            ExternalObject.AdobeXMPScript.unload();
            ExternalObject.AdobeXMPScript = undefined;
            results.message = "XMPScript Library successfully unloaded";
        }
        catch (e)
        {
            results.message = "ERROR unloading AdobeXMPScript: " + e;
            results.line = e.line;
            results.error = true;
        }
    }

    return results;
}
function XMPLibMsg (inMessage, inLine, inError)
{
    this.message = inMessage;
    this.line = inLine;
    this.error = inError;
}

Select the files, then Tools > Add-Replace-Remove Keyword

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html