Refer to the Prior Values of the Record That Triggered Your Flow

• We can compare new value , old value by using ISCHANGED  in Process Builder 
• We can do same thing in Workflow as well
• but we can't do this comparison in Salesforce Flow

In Spring 21 , we can do this comparison by using {!$Record__Prior}

Reference : https://help.salesforce.com/articleView?id=release-notes.rn_forcecom_flow_fbuilder_prior_values_flow.htm&type=5&release=230