Im posting updated script that works for me. Make sure your Paragraph style is not in folder/group of styles.
/*
_FRIdNGE-0791_SlovenianAutoNum.jsx
Script written by FRIdNGE, Michel Allio [30/01/25]
See: https://community.adobe.com/t5/indesign-discussions/changing-alphabet-in-numbered-list/td-p/15119231
*/
var myDoc = app.activeDocument;
var myParaStyle = myDoc.paragraphStyles.item("Slovenian_List"); //Your Paragraph Style name
// -----------------------------------
var myNumberingExpression = "č)\t"; //Formation of your list
// -----------------------------------
// Reset:
app.findGrepPreferences = app.changeGrepPreferences = null;
app.findGrepPreferences.findWhat = "^.+$\\r?";
app.findGrepPreferences.appliedParagraphStyle = myParaStyle;
var myParas = myDoc.findGrep();
var P = myParas.length;
for ( var p = 0; p < P; p++ ) if ( myParas[p].bulletsAndNumberingResultText.slice(0,1) === "č" ) myParas[p].numberingExpression = myParas[p].appliedParagraphStyle.numberingExpression;
// Treatment: "d" ==> "č"
app.findGrepPreferences = app.changeGrepPreferences = null;
app.findGrepPreferences.findWhat = "^.+$\\r?";
app.findGrepPreferences.appliedParagraphStyle = myParaStyle;
var myParas = myDoc.findGrep();
var P = myParas.length;
for ( var p = 0; p < P; p++ ) if ( myParas[p].bulletsAndNumberingResultText.slice(0,1) === "c" && myParas[p+1].bulletsAndNumberingResultText.slice(0,1) === "d" ) myParas[p+1].numberingExpression = myNumberingExpression;
app.findGrepPreferences = app.changeGrepPreferences = null;
alert( "Done! ..." )