You can simply go to the Component structure of the Purchased Product Structure and perform Copy Alternate and build it to the current date. This will retire the previous alternate.

@RigsIrosh I want to choose between the two alternates. So in this case, I don’t want to change the date

Yeah I guess you can set up the alternates as is. 

But generally, the alternates are introduced with phase-in/out dates, and the purchase order - supplier material or components will try to find one alternate which is valid for the given period.

Unless the user goes and edits the component lines change it to another purchase component.

Did you ever find a solution for this? We have the same scenario and I am looking to be able to either have two active alternates that can be selected between or use alternate components when shipping supplier material. Neither of these options seem to work as hoped.