A quick google suggests the knob is a separate item, and thus should be able to change easily.
https://www.ebay.co.uk/i/282280744247?ch...801&crdt=0
EDIT, a further google found a how2 on YouTube for an Insignia. I can't see changing the Astra-J knob being any different.
https://www.youtube.com/watch?v=bNDy8QS5E44&vl=en-GB
Further EDIT
It isn't by the looks of it.
https://www.youtube.com/watch?v=w9VKncS5YaI
https://www.ebay.co.uk/i/282280744247?ch...801&crdt=0
EDIT, a further google found a how2 on YouTube for an Insignia. I can't see changing the Astra-J knob being any different.
https://www.youtube.com/watch?v=bNDy8QS5E44&vl=en-GB
Further EDIT
It isn't by the looks of it.
https://www.youtube.com/watch?v=w9VKncS5YaI