How to test update routine for WordPress plugin works perfectly?

I'm writing update routine for plugin which is currently not upload on wordpress.org . the reason to write update routine is that few year back plugin with same name was uploaded on wordpress.org and i get old plugin svn credential and the member from plugins review team to write the update routine . My question how i check is my update routine is working perfect ? As there is no way i find to update the old plugin with my new plugin

via Chebli Mohamed