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 http://www.snapfiles.com/get/superscan.html 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.