Typically it is written that the project settings set for a Premiere Elements project cannot be changed after the project has been started, without having to start a new project. This situation is described in my May 23, 2015 blog post and points to Premiere Elements 11, 12, and 13. See HERE. The following excerpt from that blog post sums up the situation as it existed up to 2015 and Premiere Elements 13.
Changing The Project Preset For The Project After The Source Media
Have Been Imported And Editing The Timeline Content Is In Progress
Typically it is written that, if incorrect project settings are realized
after the project is begun, the user has to start a new project to
correct the situation. For Premiere Elements 11 and 12/12.1 we have
found a way, in some cases, to change these project settings without
having to start a new project. This is done by editing the Notepad
document of the save project file (.prel).
It was not found possible to apply this approach to Premiere Elements
13/13.1 project due to the coding that presented in the Notepad document
of the saved Premiere Elements 13/13.1 project file.
Moving forward to 2017 and version 15, the latest version of Premiere Elements, it appears that 13/13.1, 14/14.1, as well as 15, fall into the same condition, that is, it was not found possible to apply this approach to Premiere Elements
13/13.1, 14/14.1, or 15 project due to the coding that presents in the Notepad document
of the saved Premiere Elements 13/13.1, 14/14.1, or 15 project file.
I have not yet found a solution to this issue for Premiere Elements 13/13.1, 14/14.1, or 15*. I will continue to explore the topic and research any developments online by others who can read the Notepad documents for Premiere Elements 13/13.1, 14/14.1, or 15.
Suggested readings include
PE11: Changing Project Settings Without Starting A New Project
PE 11, 12, 13: More About Project Presets
Adobe Forum Thread "How To Edit Project Settings" dated Jan 7, 2016**
*Although work was done using Premiere Elements on Windows 7 Professional SP1 64 bit (Notepad, WordPad), findings should also apply to Windows 10 64 bit which was found also to offer Notepad and WordPad under Start Menu/Windows Accessories. (Windows 10 Home 64 bit was the actual computer.)
**Reference is given as one example of many of my discussions on topic. I cannot be reached at Adobe Forums for follow up discussion of that thread since I no longer post nor answer questions there.