I've used this program myself and I can say that it is safe and it does work, IF you follow the directions. To remove it from the 28 pages that the other thread is currently sitting at, this will be posted at the top of all forums. Please direct all questions to either this thread or the Homebrew thread that will be joining it as needed. Thanks.
Downgrading from 2.0 to 1.5
The following two files are necessary...
1.50 Update EBOOT
This will ONLY WORK on a 2.0.
1) You need to unzip the files MPHDowngrader.zip and EBOOT_1.50.zip. Use a program such as WinRAR, 7Zip, or WinZip (check Google). Once they are unzipped, you're ready to begin.
About Virus Warnings
Shortly after the release of the downgrader, the same exploit which makes this possible was used in the creation of malicious code; antivirus software have since started to block the downgrader files from extraction. If you cannot extract all files, you will have to disable antivirus for the duration of the downgrade process.
2) Set up a USB connection with your PSP and your computer.
3) Once the connection is set up, open up My Computer and browse to the removable disk that is your PSP. You'll need to set a few things up here.
4) When you first double click to open up the PSP on your computer, you'll be in what's called the root menu. There will be a PSP folder showing if you are in the right place.
5) Copy over to your PSP the following files to THIS location in the root folder...NOT in the PSP folder or any other folder. Index.dat and h.bin should both be in your root directory.
6) Once this is done, open up the PSP folder and then open up the Photo folder. Here, you will copy overflow.tif over, keeping it in the Photo directory and not a sub directory.
7) Now, go back to the PSP directory and go into the Game directory. If there is not already, create a folder called UPDATE. It MUST be called UPDATE and nothing else.
8) Open up the new UPDATE folder and then copy EBOOT.PBP into this folder from your computer. Once all of this is done, you're ready to downgrade. So far, so good.
Double check that you followed the above instructions before continuing.
9) Now, disconnect the USB connection from the computer and plug the AC adapter into the PSP. As with all upgrades, you must have a certain percentage of battery life as well as the AC adapter plugged in, or this will not work.
10) Go to the Photo icon on your PSP and scroll through the pictures to get to overflow.tif. You do NOT have to press anything. It will start automatically. Two strings of letters/numbers will appear. This is normal and good.
11) Wait AT LEAST 30 SECONDS and then turn off your PSP by holding the power button up until it fully shuts down. Then, start your PSP again.
12) You will now go over to the Game icon and scroll down to memory stick. Select it and you should see the 1.5 update.
13) Select it and start it up. Go through it as you've done before with your 2.0 update and follow the prompts. When the update reaches 99% it will crash and show an error. THIS IS OKAY. DO NOT PANIC. Once again press the power switch up until it fully turns off.
14) Now, turn the PSP back on. You will get another error. Just press the "O" button and it will restart after rebuilding the system data. Then just set it up as you did when you first got your PSP and you have a brand new 1.5 PSP. Be responsible with it. Once complete, you may remove the downgrader related files from the memorystick.
If you have any concerns, please check this again before going elsewhere due to being extremely busy. Good luck and be careful with this.