Not possible.

The current status of exploit/hacking with 2.5 PSP is that there is no exploit available yet.

The main problem is that all new "executable" files of a game/update are encrypted. The PSP knows how to decrypt, only Sony knows how to encrypt.

So we are able to decrypt using some homebrew apps as of today and checkout some files of the firmware updaters, but we can not encrypt our homebrews or exchange files within the firmware updaters (make a fake 2.51 which is the 1.5 updater for example) as we would have to encrypt these files again in order for them to be run.

If an application/game is not encrypted, the PSP wont run it.

So:

- No exploit to set the PSP version to 1.00 and use a 1.50 update
- No way found to encrypt software yet (sign the app) so the PSP accepts and runs it

However, a modchip should appearantly disable the "encryption requirement", though is not available yet. Additionally, some programers are working on ways to find out the encryption keys as we now have encrypted and decrypted versions of a file available.