I have tested this on my p500 and it worked. I downgraded to v10b version 2.2.
downgrade your firmware
If you want to root your phone then follow this guide. Rooting LG Optimus One P500 using GingerBreak

Step 1

Method 1:
Download the firmware also known as rom from Lg’s firmware repository. The following link will list all the country codes for LG optimus p 500. http://csmg.lgmobile.com:9002/csmg/b2c/client/ezlooks_model_list.jsp?model=LGP500 (if this link doesnt work then try method 2)
Press ctrl + f on the keyboard to bring up the browser search tool and type the name of your country. Your country code will look something like this. Don’t get scared after seeing the codes. It displays only codes supported by lg p500. Choose your country specific code. You could also choose other country codes if you wanna experiment.
country codes lg optimus one p500

or

Method 2:
Follow this method if method 1 doesn’t work. Skip step 2 & 3
Here’s a list of the latest unbranded firmware as of 9/03/2011 4:15 EST
Just click on the links below to start the DOWNLOAD
http://forum.xda-developers.com/showthread.php?t=865479&page=18
LGP500 – AAUSBK – AUS/AUSTRALIA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10B_00.kdz
LGP500 – ABALSV – BAL/BALTICS
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10E_00.kdz
LGP500 – ACISSV – CIS/CIS
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – ACZEBK – CZE/CZECHO
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – ADEUBI – DEU/GERMANY
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10E_00.kdz
LGP500 – AESPBK – ESP/SPAIN
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – AGBRBK – GBR/UNITED KINGDOM
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10A_00.kdz
LGP500 – AGLOBK – GLO/Philippines GLO
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10B_00.kdz
LGP500 – AGRCBK – GRC/GREECE
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – AHKGBK – HKG/HONG KONG
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10B_00.kdz
LGP500 – AHUNBK – HUN/HUNGARY
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – AINDBK – IND/INDIA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10D_00.kdz
LGP500 – AIRQBK – IRQ/IRAQ
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10D_00.kdz
LGP500 – AITABK – ITA/ITALY
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10E_00.kdz
LGP500 – ANEUBK – NEU/EUROPE OPEN
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – ANGRBK – NGR/NIGERIA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10A_00.kdz
LGP500 – AORIBK – ORI/ORANGE ISRAEL
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – APOLBK – POL/POLAND
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10H_00.kdz
LGP500 – APRTBK – PRT/PORTUGAL
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10E_00.kdz
LGP500 – ARUSSV – RUS/RUSSIAN FEDERATION
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – ASGPBK – SGP/SINGAPORE
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – ASLOBK – SLO/SLOVENIA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10D_00.kdz
LGP500 – ASWSBK – SWS/Switzerland
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10D_00.kdz
LGP500 – ATHABK – THA/THAILAND
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – ATWNBK – TWN/TAIWAN
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10C_00.kdz
LGP500 – AVNMBK – VNM/VIETNAM
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10F_00.kdz
LGP500 – AZAFBK – ZAF/SOUTH AFRICA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10A_00.kdz
LGP500 – ASEABK – MALAYSIA
Newest found : http://csmg.lgmobile.com:9002/swdata…00/V10A_00.kdz
Step 2
Copy your country’s code. For example for india its AINDBK. Now paste it in the url below where it is marked XXXXXX.
http://csmg.lgmobile.com:9002/swdata/WEBSW/LGP500/XXXXX/V10b_00/V10B_00.kdz
Your link would look something like this after pasting the code
http://csmg.lgmobile.com:9002/swdata/WEBSW/LGP500/AINDBK/V10b_00/V10B_00.kdz

Step 3

Now replace the V10c_00/V10C00 with the version number you want. For example if i want to downgrade to version 10B then i would change the letters from c to b like this V10b_00/V10B00.
Now copy and paste the link in the addressbar of your browser and press enter key. Download should start. If it doesnt then you have made a mistake. Construct your link again by following the steps above.

Step 4

After the firmware download is finished. Download firmware updating software KDZ_firmware_updater and extract it to a folder.

You will have the following files. Install msxml on your windows pc by double clicking it.
**Now follow these steps carefully
  1. Connect your phone and run KDZ_FW_UPD.exe without turning on USB storage on your phone.
  2. In the software window, select Type as 3GQCT
  3. Select PhoneMode as DIAG
  4. In the place where you have to give path for KDZ file, select the Firmware a.k.a ROM you have downloaded using the link you made for your country specific firmware.
  5. Press Launch software update and wait for about 10 minutes. The screen will then say “Emergency mode”. Just wait and let it reboot.
kdz updater lg
Dont unplug the USB cable until the setup has finished.
After everything has finished, switch on your phone. Your phone will have the firmware you installed!
Disclaimer: I cannot be held responsible if you brick your phone. Do it at your own risk!