Sql Server Agent Job - Unable to start execution of step 1 (reason: Error authenticating proxy "user", system error: The user's account has expired.)

My predecessor created several "Intergration Services Catalogs" dtsx packages which are scheduled / run via SQL Server Agent -> Jobs using an SSISFileProxy which is using SSISProxyCredentials. The expired error "user" is referring to my predecessors user account, and the only object I have seen that uses his account was in the Job's Properties -> General -> Owner field. I changed the Owner to another server admin but after rerunning the job, the error message still referred to the old expired user account (rather than the updated Job owner). I am new to MSSMS and am struggling how to even articulate my question, but assume there is a simple solution. Any assistance you can provide will be greatly appreciated. Thanks!