How do I change the date that divides current and historical transactions for all customers?

Transactions get moved to "history" when you perform a year end function in AR.

Actually, running period end can be used to move current transactions to history, and this is my recommended method. Year end can also be used for this purpose but I usually find that long period year end comes around there are too many transactions sitting in current.

Only fully paid transactions are moved to history by either period or year end.