there is no need to downgrade. you have exactly what you need to install chickHEN.
you can download chickHEN r2 from here:
ChickHEN Homebrew Enabler R2 for Frimware 5.03 | HEN for PSP-3000, PSP Hacks - PSP Slim Hacks
the instructions for installation can be found in the readme file.
once you have chickHEN installed, you need to run a program called pspident to figure out which motherboard your psp has installed. its very easy.
you can download pspident here:
PSPident 0.4 - Find out what PSP Motherboard you have., PSP Hacks - PSP Slim Hacks
again, the instructions are in the readme file. after your know which motherboard your psp has installed, then we can proceed to the next step.