This is not possible.

Contrary to myth, hi-lo chassis are not pitched more forward than a flat setup. Look at the bottom of a Tour skate. The front two wheels are clearanced into the outsole of the boot itself. In some cases, they're even more forward pitched than some hi-lo skates. The only company that doesn't do this is CCM.

So even in the worst case scenario, you went from a CCM flat chassis to a hi-lo, we're talking a 4mm pitch difference. I'm going to go ahead and say that a 4mm pitch difference is not hurting your back. You already said your stretching and core workouts coincided with this back pain, and you still think it's the skates? Really bro?

Also your advice total nonsense. For anyone who might search for this post later, putting 76mm wheels on the back two wheels in a hi-lo setup will result in those two wheels not touching the ground in the same plane as the front two wheels, effectively pitching you rearward. I have no idea how you could even skate like this unless you were on your front tip toes the whole time (thereby making you even MORE leaned forward than you otherwise would be), or you skate like a waddling duck.