It is generally recommended to finish a project in the version which it was created.
Save a copy and work with the copy if you feel the need to try the project in version 13.
What is the computer operating system involved?
Are Premiere Elements 12 and 13 on the same computer? If not, please name the two computer operating system?
Premiere Elements 12 and 13 are very similar. Is there any particular reason why you want to complete the 12 project in 13?