Salesforce Opportunity Probability Not Updated When Stage Changes to a New Custom Value

Why Existing Opportunities Are Not Updated

The Salesforce Probability field is directly linked to the Stage value but behaves as a default rather than a locked value. Because the probability was already saved on the existing record before the stage configuration change was made, those records retain their previously saved probability value.
Example: The stage Closed/Won has a default probability of 100%. A user with edit access to the Probability field can manually change this to 57% on a specific Opportunity. If the administrator later changes the default probability for Closed/Won from 100% to 95%, existing Opportunity records in Closed/Won status will continue to show whichever probability value was previously saved — whether the default 100% or a manually overridden value like 57%.

How to Trigger the New Default Probability

Manually changing the Stage on an existing Opportunity will cause the new default probability to populate. The record must be actively edited and the Stage value re-selected for the updated default to apply.

How to Update Probability on Multiple Existing Opportunities

To mass-update the probability value across many existing Opportunity records, use a data update tool such as Salesforce Data Loader or Data Import Wizard. Export the Opportunity records that need updating, modify the probability values in the export file, and re-import the file to update those records.
For detailed guidance, see Mass update newly changed Probability (%) value for Old/Existing Opportunity records.