You can directly edit the code of the assignment strategy to do almost anything, including choosing between two strategies depending on, in your case, the model time.
Click on the scroll icon to open the code editor.

Copy the relevant parts of the code to the clipboard (everything but the first three lines, where the parameters are defined)

Close the window again and choose the second assignment strategy. Then open the window again. The code will have changed to reflect the change.
Paste the other code in behind. Then enclose the two blocks with an if-condition based on the model time.

Lastly you can (but don't have to) remove the light grey text which is used to generate the pick option GUI for the assignment strategies. To do this click on the icon in the lower left corner.

This will remove all markup and add the description 'custom code' to the first line. You can edit this to something more descriptive which will then be displayed in the slot assignment field.
