You can use the getter/setter alternative syntax and override the getter/setter functions in the extended scripts:
Example:
extends Node
var my_var:float: get = get_my_var, set = set_my_var
func _ready() -> void:
my_var = 5
printt(name, my_var)
func get_my_var():
return my_var
func set_my_var(value):
my_var = value
Extended script:
extends "res://script.gd"
func get_my_var():
return my_var * 2
It will print:
Child 10
Parent 5
as Child
has attached the extended script.