It is possible to create your own switchover functionality.
This will require the use of some IF statements to indicate the conditions for when the switchover should occur.
To do this you need to create a module which stores the time period setting for when the switchover should take place.
This module should contain a single line item with no dimensions and a time period format. In this line item you can select which period you wish for the switchover to occur. Call the line item 'Switchover Time Setting'.
Then create a module dimensioned by time and which includes two line items both Boolean formatted. One to indicate the boolean for the switchover version and one to indicate the boolean for the actual version.
Actual version? line item formula IF ITEM(TIME) >= 'Switchover Time Setting' THEN TRUE ELSE FALSE
Switchover version? line item formula IF ITEM(TIME) < 'Switchover Time Setting' THEN TRUE ELSE FALSE
Then in the module where you wish the switchover to happen you then reference the booleans from the module described above.
Such as IF Actual version? then Your Actual Data[SELECT: Dummy Actual Version] ELSE IF Switchover version? THEN Your Forecast Data[SELECT: Dummy Forecast Version] ELSE 0
Also instead of selecting the fixed versions you could create a module where there are two line items 'Actual' and 'Switchover' with the version list format and then set which is the actual version and which is the switchover version. And then make the SELECT references in the formula above to reference these 2 line items instead giving you flexibility to change the versions for the switchover to occur.
I hope this helps and if you have any questions please do let me know.