Way to quickly replace the marker text in all same custom markers

Hi Klaus, here is some starter code for you (untested):

main ();

function main () {
	
	var doc;
	
	doc = app.ActiveDoc;
	if (doc.ObjectValid () === 1) {
		processDoc (doc);
	}
}

function processDoc (doc) {
	
	var marker, markerType;
	
	// Get the change-to marker type.
	markerType = getMarkerType ("newMarkerType", doc);
	if (markerType.ObjectValid () === 1) {
		// Loop through the markers in the document.
		marker = doc.FirstMarkerInDoc;
		while (marker.ObjectValid () === 1) {
			if (marker.MarkerTypeId.Name === "oldMarkerName") {
				marker.MarkerTypeId = markerType;
			}
			marker = marker.NextMarkerInDoc;
		}
	}
}

function getMarkerType (markerName, doc) {  
     
    // markerType = getMarkerType (markerName, doc);
    // Get a marker type from the document; if it doesn't exist, create it.
    
    var markerType = doc.GetNamedMarkerType (markerName);  
    if (markerType.ObjectValid () === 0) {  
        markerType = doc.NewNamedMarkerType (markerName);  
    }  
    return markerType;  
}

IGOR5C0C: Could you clarify your proposal with a simple example?

MIF is Marker Interchange Format. It's a save-as option that renders a .fm (or .book) as plain ASCII in a legacy FM markup notation.

I created a default portrait document {mif-marker.fm} with some random body text.
I used Insert Marker to create a custom marker of name CustomMarkerName
and with marker text of 17.11.2021. I inserted another after creating the marker type.
I then saved the document as MIF to mif-marker.mif
and opened that with a plain text editor (notepad).

The body text instances of the marker look like this in the MIF:

<Marker

 <MType 19>

 <MTypeName `CustomMarkerName'>

 <MText `17.11.2021'>

 <MCurrPage `1'>

 <Unique 998427>

> # end of Marker

If this is similar to what's in the MIF for your documents, it might be sufficient to:

1. Save all the .fm & .book as mifs
2. Open the mifs in an editor
3. change all instances of
   <MText `17.11.2021'>
   to
   <MText `18.12.2022'>
4. Save the revised files (still mifs)
5. Open the mifs in FM, and if no problems…
6. save-as back to .fm or .book

I used to write Unix sed scripts to batch similar changes across huge books.