Quoted from noobz.eu:
" A long time ago, when the GTA exploit first emerged, the first application we wrote for it was a v2.01 downgrader, similar to MPH's downgrader for v2.00. Unfortunately, the GTA exploit didn't offer sufficient access to the PSP's flash to allow the downgrader to work, so it was never released.
The new TIFF exploit allows us the necessary access, and so here it is, after some thorough testing on our own PSPs - an easy downgrader for firmware v2.00 and v2.01, via eLoader 0.9.8.
The advantage of this downgrader is that it doesn't require a firmware dump from a v1.5 PSP. It also contains a lot of extra safety checks and user friendliness enhancements, as well as a safer method for overwriting the INDEX.DAT file.
Many v2.0 and v2.01 users who wanted to downgrade may already have used the v2.6 or v2.71 Downdaters, so the usefulness of this downgrader is perhaps limited. But we're releasing it now so that anyone who might find it useful can use it.
Please ensure that you read the entire README.TXT file that is included with the downgrader package. Please also note that although this uses tried and tested techniques, and has been tested with 100% success on several PSPs, no downgrader is 100% safe, and so YOU USE THIS SOFTWARE AT YOUR OWN RISK."
Now we only need a downgrade for 1.51/1.52 and we will be all fine and dandy!
Link to the site: http://www.noobz.eu/
You can also download it there as well.