_layer = iface.activeLayer()
a = _layer.sourceCrs()
b = QgsCoordinateReferenceSystem("EPSG:5186")
_layer.setCrs(b)
mapCanvas = iface.mapCanvas()
extMap = mapCanvas.extent()
transform = QgsCoordinateTransform(a, b, QgsProject.instance())
extMap = coordTrans.transform(extMap, QgsCoordinateTransform.ForwardTransform)
mapCanvas.setDestinationCrs(b)
mapCanvas.freeze(False)
mapCanvas.setExtent(extMap
