By Enterprise you mean SOAP, code generated out of "Enterprise WSDL"? SOAP should still work OK, what error you're getting? What's in the connecting user's login history?

Are you calling generic login.salesforce.com/test.salesforce.com or the proper "My Domain"? Did you consume a WSDL (https://trailhead.salesforce.com/content/learn/modules/api_basics/api_basics_soap and https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_quickstart_steps_import_wsdl.htm) or by JAR you mean say DataLoader's JAR file?

"What changed"... New objects introduced. OAuth username-password flow disabled by default (not that you should care). How's your session id generated?