Rules as written, changes to Toughness don't affect max Hit Points after character creation. I also checked Mork Borg core, and it seems to be that way there as well.
That said, I'm completely okay with Toughness changes affecting max HP.
There *is* the possibility of a weird edge case where someone with very low max HP but moderate Toughness could lose a point of Toughness and hit max HP zero before they even reach negative Toughness, but I'm not sure how often that's likely to come up.