By default, the Validity Period of new Version for any Modification of Product Structure is unlimited. So, both Dates ("Valid From" and "Valid To") are Empty. If only one single Version for Modification of Product Structure should be used in production, then the “Valid From” and “Valid To” fields can be kept as Empty for the all time. It means that this unlimited Version can be approved and will be used to create any new Production Orders for any time in future. If necessary, the Components List of this Version can be changed.
How to be when You need to change Product Structure for particular Inventory Part PERIODICALLY? For example, the previously used Components cannot be used anymore beginning from known Date, because You need to change previous Supplier to the new one (together with new Inventory Parts, new quality of purchased raw materials and components, new Prices etc.). The usage of “old” Components can be just prohibited by new ecological Laws. Moreover, the dire necessity to change Components List can be caused by changed technological process (Routing) for the Product (e.g. due to adoption of new or improved technological equipment).
The answer is very easy: You have to create a FEW Versions for Modification of Product Structure and prevent their overlapping in time. In other words, You need the possibility to define Validity Period for each Version of Product Structure.