Signoff Authority Transfers
You can used transferred authority from you to another user or from another user to you. This is useful when, for example, you are out of the office and unavailable to sign off on changes.
To view transfers that have been set up:
1. Open your user profile by clicking the My Settings button in the left navigation pane.
Your user profile appears in the content pane.
2. Click the Transfer Authority button.
3. The All Transfer Authority Details dialog appears.

as described by Hagi, the Transfer Authority is the best way to do that.

From the documentation:

The Transfer Authority for Others privilege allows an authorized user to transfer an original approver‘s right to approve or reject changes to another user for a specified period of time. The authority transfer applies when the original user is an approver on a routable object, but not an observer.
This privilege overrides transfers designated by users who have the Transfer Authority for Self privilege. The original approver is copied on all notifying email.
Note When you create a transfer criteria, you can include criteria conditions with Affected Items tab fields. To select a specific routable object, an Affected Items field condition must be true for all the objects on the Affected Items tab of that routable object.
For example, if you specify Affected Items.Old Lifecycle Phase Equals Preliminary, then all the objects on the Affected Items tab must have the Old Lifecycle field equal to Preliminary. If you select Contains as the search operator, then every object on the Affected Items tab must contain the specified value in the specified field.

Defining custom criteria that match your requirements, you will be able to select different users as “delegated” approvers.

Otherwise, you can develop a Mass Approvers replacer script via SDK to replace permanently the old approver with new one