I just updated to picard 2.7.3 on an old (mac os) computer and also installed it on a new one.
I had a modified file renaming on the old computer and so have a “user” file naming script option (in addition to the three presets) which I am able to edit.
On the new computer however I see the three preset options but they all appear to be read only and I see no obvious way to create a new one from scratch or clone an existing one. The documentation doesn’t seem to make this clear.
TBH, I not used it before. I think what you need is hidden in a menu.
OPTIONS \ Open File Naming Script Editor…
Then go to SCRIPT \ ADD A NEW SCRIPT
You should be able to then paste in your old script.
Same menu also has a “Copy the Current Script” option.
If you open the File Renaming Script Editor either from the Options section of the Main Screen menu or from the Options → File Naming page, there are options in the menu to add a new script (based on the default), copy the current script (for editing), as well as export the current script to a text file and import from a text file.
In your case, since you have the script still on the old computer, I suggest that you export it to a text file. Then copy the file to your new computer and import it.