What says it can go vice versa? Clearly, Hinata failed to awaken the Tenseigan, so that alone proves the process cannot be reversed. Meaning Hyuuga can't awaken the Tenseigan, even if they somehow manage to obtain Hamura chakra, since Hinata herself failed with all that hype about her being the Hyuuga princess.
But we do know that Otsutsuki can through some means awaken it if they manage to get hold of a pair of Byakugan from a pure Hyuuga.
Sadly, the Hyuuga clan is dying out since they've started to intermingle with other clans. So they're pretty much diluting the clan's purity and eventually it's going to be impossible to awaken it, for Otsutsuki that is. Since Hamura's Byakugan descendants will have been sullied.
Just my opinion though, but I don't agree with Otsutsuki being supposedly certain to awaken the Tenseigan. There might be a secret to it, but it wouldn't make sense if you got all of that power out of just acquiring a set of Byakugan eyes. It's just absurd to think about.