cant extend UnitAmountCur (Microsoft says its extendable)

Hello friends,

I need to change SalesPrice in SalesLines table up to 5 decimal numbers. As I understand you can do it by extending right EDT type on which SalesPrice is based on (Microsoft official docs screenshot in attachment), that is - UnitAmountCur EDT. However, when I try to extend it in the project its not possible (screenshot 2 and 3 attached). UnitAmountCur is based on AmountCur, which is extendable in the project, however its referenced in lot of other places and can seriously harm the project in other places. Do you have any suggestions? Have Microsoft changed UnitAmountCur in one of their patches and its not extensible anymore? It would be nice if you could help me in this.

Best regards,