Changing 'Amortization Start Date' to a previous closed period date

We are looking at changing the useful life for a number of assets and as part of calculating the depreciation, we want to change the Amortization Start Date to 01-Jul-2018 since it represents the start of the financial year.

At this stage, when I go into an asset's books and click on the 'Amortization Start Date' field, it automatically defaults to the last day of the current open period (which is 31-Jan-2019 in our test system).

The following note advises that we have the option of changing the default amortization start date to a date in the previous period.

Does anyone know how this can be done?