One Chinese said he has succeed to downgrade v1.50 version PSP to 1.00...
1. You need a WIFI card, for example a DLINK DWL122.
2. Set your dwl122's IP, then you must set PSP's NETWORK SETTINGS as INFRASTRUCTURE MODE and SSID is your dwl's IP, and then go to PSP's GAME SHARING to connect your PSP and PC, or you can use KAI to try.
3. Then you must use port-scanning software such as SuperScan 3 to scan the port which your PSP has. Usually it's in 31XXX~62XXX. The English version of SuperScan 3 is HERE for download.
4. If you have successfully scanned the port, you can login to your PSP in Internet Explorer for example: http://192.168.1.1:31413
Then you can see the file folder in your IE. There is 2 pics attached below this post. Finally, you can replace the folder, but you can't replace "data" files. I think it's possible to use the PSP v1.0 version firmware in circulation, but it doesn't include the installer.
I'm sorry by my poor English... read on another forum and am trying to download and try myself.
I managed to find a port but couldnt connect on internet explorer my port was 45100 i used Advanced LAN Scanner make sure you go to options and select port range otherwise it only scans 66 ports but i did find aport just couldnt go any further
another thing with downgrading... you are using the PSPs firmware to have your computer connect to it. im not sure with the psp, but i kno with computers u cant overwrite a file while it is in memory. does this hold true with the psp?