PSP doesn't support or create PNG Plus files, though it's DIP and FW native format. The best format for PSP is the .pspimage or just plain .psp to support layers and make editing easier. The standard PNG doesn't hold layers, and PNG Plus is a diferent file extension.
Once flattened you canNOT go back! The Clone tool is the best, then type over the top.
Save 2 or 3 copies of each layout
1. native format, layered origina size (.psd Adobe, .psp PSP, etc)
2. small size, low quality .jpg file for sharing on the web/email
3. optional original size large quality .jpg file for sending to pro printers/photo developers
So while there's no way to 'relayer' your layout, you didn't flatten very many and learned a free lesson!
I always have to remind myself that!