Hi Remco. It is very easy to do by using Joomla Custom module and a small CSS coding.
The starting point for this:
You can publish Custom module containing another logo in the Logo position.
Then, using CSS you need to hide this module in the state before scrolling and display in the state after scrolling.
And hide main Logo in the state after scrolling.
Parent classes that you can use:
/*Before scroll*/
#sp-header .some_kind_of_child_class {
/* your code goes here */
}
/*After scroll*/
#sp-header.header-sticky .some_kind_of_child_class {
/* your code goes here */
}
With this you can change anything inside Header before and after scrolling.
Or you can use JavaScript to change the value in the src attribute of .logo-image after scrolling.