You cannot save a change in a combo box property unless you change it
in Design View.

I would suggest you create a table with one field and one record.

Field Name    NewDefault      Set it's datatype to whatever the
datatype is of the combo box bound column.
Table Name   tblDefault

to store whatever the last selected value is (in the Combo AfterUpdate
event)

' First change the default value for use while this form is still
open.
Me!ComboName.DefaultValue = Me!ComboName

' Then save the value to the table.
CurrentDb.Execute "Update tblDefault Set tblDefault.NewDefault = " &
Me.ComboName & ";",dbFailOnError

The above assumes the bound column is a Number datatype.

Then DLookUp that value when you next open the form (in the Form's
Load event):

Me!ComboName.DefaultValue = DLookUp("[NewDefault]","tblDefault")

You do know, I hope,  that the DefaultValue will only come into play
for new records?