We know there is an afterlife, it's what happens after we have died. The body rots.
Somehow I think this is not what you mean. But if we somehow 'live' after life, than it's obviously not an 'after'-life. It's still life.
I don't want to ask how the existence of an afterlife/reincarnation could be established. Let's assume that it has. Well, the adjustment in belief would be this: before the discovery of the existence of an afterlife/reincarnation a humanist had no reason to belief in the existence of an afterlife/reincarnation, after the discovery he has.
Right now I believe that our planet has one moon. This is what the experts are saying. If the experts some day announce they have discovered a second moon (maybe eclipsed by the visible moon), I will have to believe them. I will adjust my belief, because expert in the field have used reason and logic to come to this surprising conclusion. If later they revise their opinion, I will adjust my belief again.
Where it gets interesting is when we have answers to questions surrounding these phenomenon:
How do we get there?
Will everybody get there?
Can we get back here?
What's it like?
What are the rules?
Is there only one place, or multiple?
Where does it all take place?
Do we keep our bodies, or de we get new ones, or do we not have bodies?
What are we doing, and how are our activities fuelled?
Do we need to eat, sleep, drink, use the toilet?
Where does the food come from, where does the waste go?
If afterlife is better then the before-afterlife, then we should get there as soon as possible, whilst our bodies are young and healthy. If it's a nasty place, maybe we can avoid it altogether? So much depends on the answers to the questions above, and many more.