Is this even possible???

I have my main site at http://www.mainsite.com I copied that site using the duplicator plugin and installed it at http://www.test.mainsite.com in order to upload a new theme and make it look like my main site. I am now finished and I am struggling with the best way to make this site live and replace my main site. Some say to use the duplicator plugin but I’m not exactly sure how to upload the package.

I was wondering if it is possible to go to my main site say http://www.mainsite.com and change the url in the wordpress dashboard to http://www.mainsite.com/old and then go to my test site at test.mainsite.com and change that url to http://www.mainsite.com. Then use the velvet blue plugin to update the urls for images etc.

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