Where or how do I have to change the isin? After much trial and error I have managed it. In edit mode, select the name and isin for all purchases and sales and insert the new isin. The display is now correct again!
I was also wondering @Komar did it show the split correctly after you updated the ISINs? I've held the shares since 2017 and there have even been two splits since then... If it doesn't adjust, it would look like I've made losses