To change what Approval Scheme is attached to an Agreement, go to Agreements, locate and click on your Agreement so you see the below page layout:

 

Under the Approval section, click on the cog on the right:

 

You will then see a list of the available Approval Schemes:

 

The tick will indicate the current Approval Scheme attached too this Agreement. By clicking on the  button you will replace the existing Approval with the newly selected one:

 

 

Once selected, click the Done button and it will take you back to the Agreement page and the Approval section will now show its new Scheme