Once a SAP accepts the Designation, you are locked in to them.

Once you send the designation request from your Clearinghouse account, the SAP will be notified that someone has requested their acceptance of your request. When the SAP logs into their Clearinghouse account it will have a button which will either accept or deny your request. Once they accept it will show them being designated. 

As an employee, ask your employer for a list of SAPs in your area  (even if the employer fired you or did not hire you).  A SAP qualified  to act in the DOT drug and alcohol testing program must meet the SAP  qualification requirements in §40.281. MOST OF THE TIME EMPLOYERS WILL NOT PROVIDE YOU WITH A LIST OF SAPS.... 

If your employer did not provide you with a list of qualified SAPs, below is one way to help you find one:

• Search the internet using  search terms such as “DOT qualified SAP” or “DOT and SAP”

Please know that neither the DOT nor any of its Agencies has a list of SAPs or any other service agents.

Until your SAP accepts the designation you are actually still able to change the SAP provider as long as it still shows pending you can change the name of the SAP. Once it says "DESIGNATED" you are unable to change the SAP on your own, you would have to call FMCSA Clearinghouse and have them changed by HQ if they do accept it -- AS LONG AS THEY HAVE NOT COMPLETED THE INITIAL ASSESSMENT.