They wouldnt press chages against the user they'd do i against the writer. It's the distribution of a hacked updater they'd have the issue with not the fact you have one on your PSP. Basicly, if it is a hacked updater, my guess is that a patcher will be released for the v2.0 updater, which will replace the contents of the patcher with the contents of the v1.5 updater. None of which would be illegal.
So you'll probably need to download the v2.0 updater, the v1.5 updater and the downgrader program. Then you'll probably run the downgader and it'll take parts from the v1.5 and put them into the v2.0. Effectively turning the v2.0 updater into a v1.5 updater, but still making PSP think that it is a v2.0 updater.
Then when you run the v2.0 updater it'll actually downgrade your PSP to v1.5.
This may be totally waaaay ff but i reckon this'll be how it works.
Thus at no stage is it illegal, and easily distributable on every reputable site.