Get back the old Google+ URL

You pretty much can’t. Not at the moment anyway.

Your custom URL can’t be changed. It will be preassigned based on things like your name or the name of your linked website. You may also need to add a few letters or numbers to make it unique to you.

Source.

To be able to reuse a site URL, the original site should be deleted permanently first. To make that this be applied to all the Google Servers could take a while, so wait few hours, better, one or two days.

It's worth to say that Google Sites allow to have multiple owners, so it's not neccessary to delete a site when it was created with the wrong account, instead just add the right account as owner.

From Share your site with other people - Sites Help

Anyone you set to Is owner can change the look and feel of the site as well as make administrative changes, such as deleting the site or adding new owners.