Changing a regular album into a smart album?

You can't change a regular album https://support.apple.com/en-in/guide/photos/pht6d60a1f1/5.0/mac/10.15

To smart albums created https://support.apple.com/en-in/guide/photos/pht6d60ca71/mac

Both have separate methods to create .

Send your feedback https://www.apple.com/feedback/photos.html

What kind of smart album do you need? Do you want to filter the photos in the standard albums by applying further rules?

There are two ways to usethe photos from a standard album in a smart album:

• Use keywords:
  • Select all photos in the standard album at once with ⌘A, open the Info with ⌘i then add a keyword with the name of the album in the "Add a keyword" section of the Info.
  • Now create a new smart album with the rule "keyword is ..." and select the new keyword with the album name.
• Use the album directly:
  • Create a new smart album with the rule "Album is ..." and select the album name. All photos from your standard album will appear in this smart album. But you have to keep the standard album around.

I'm not sure what that would mean. A Smart Album contains only items that match a rule or a set of rules, which in principle could be a different set of photos every time you display it.

Just delete the "regular" Album and create a new Smart Album of the same name with whatever criteria you want. That would be indistinguishable from changing the regular album to a smart album.