There's always a risk.
when you downgrade, you use code made by a few persons. This code, even though it's been tested a lot, didn't go through a QA process, and, after all, was never validated by Sony (obviously).
If the downgrader is perfect without a single bug, downgrading/upgrading several times should have no impact on your machine. But code without bug does not exist.
Most likely, if there is a risk to mess with your nand, the number of times you downgraded is unrelated, you mess it up the first time you downgrade, and you run a different risk every time you use some different software coded by some different dev.
For the anecdote, I downgraded my phat in 2006 to install a CFW. I was then unable to install OFW for more than a year, until DDC 7 (or was it 8?) added an option to re-create the nand.
That specific PSP (a phat) cannot connect to the PSN anymore, even if I install OFW on it.
When I say downgrading is dangerous, I mean it. People might believe that they downgraded without a problem, and tell everyone that it works, and next time they try to watch a UMD, or connect a usb cable, or play online, they will realize they can't install OFW anymore, or that even if they install it, their PSN account is rejected, etc...
the (bad) impacts of downgrading can be felt years after you downgraded... As far as I'm concerned, I realized my phat was scr..ed in 2010, because of a downgrade I did in 2006...
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