how can I change a regular photo album to a shared album without starting a whole new shared album with same photos

You cannot, if you share an already created album under "my albums," it will create a new shared album under "shared albums."

Open Photos

Select the album that you wish to share

Click on the share icon (Box with the arrow)

Create a new shared album or add the contents to an already created album

It is not possible, because Shared albums are storing their own copies of the photos. Photos is creating special versions for sharing - reducing the size, removing metadata.