Run territory assignment rules when you edit an Account

There is a checkbox that can be added to the Account page layout called "Evaluate this account against territory rules on save. 

Steps to Add Territory Checkbox to Page Layout

1. Setup -> Customize -> Accounts -> Page Layouts

2. Click Edit next to the page layout

3. Click the Layout Properties button at the top of the page

4. Check the "Show on edit page" checkbox next to "Evaluate this account against territory rules on save"

5. Optionally check the "Select by default" checkbox to auto-check the checkbox on the page layout

6. Click OK then save the page layout

Note:  Make sure Territory Model is Activated prior to changing the Territory Number(if Custom Territory Number field was used) for the Account Territory record to be available in Assigned Territories Related List.

The only option as of API version 21 is to use the Web Services API. See AssignmentRuleHeader in the Web Services API Developer Guide for more information:

Limitations

"AssignmentRuleHeader – useDefaultRule - If true for a Case or Lead, uses the default (active) assignment rule for a Case or Lead. If specified, do not specify an assignmentRuleId. If true for an Account, all territory assignment rules are applied, and if false, no territory assignment rules are applied."

Note: If a territory has multiple locally-defined account assignment rules, an account is assigned to the territory only if it matches all locally-defined account assignment rules on the territory.