When I have run into this I had to reopen the order and maybe the lines too… 
I can’t remember. But I’m not saying that is what you should do… I don’t know the business logic around this and I’d log a support case with Epicor to give you more info around the error you are getting.
Can I do that even if it is already invoiced (forgot to say this sorry)
Thank you so much for your help
Well I think, with your information I can now work with my Epicor on this (and it will seem I was realy trying before asking) , because I think I will need some permits to do this.
I would ASSUME you can… it shouldn’t break any of those links to invoices… but, I can’t give you a guarantee on that one.
I did this after importing old legacy Sales Orders into the system which weren’t shipped/invoiced from within Epicor.
When in doubt, test it in your test/pilot environment first.
Ok, Thank you very much, I’ll try all your suggestions for sure. 
FOR SURE!
I will