I’d say that you should upgrade your bike when you feel as though your current set-up is a limiting factor, and when you’re willing to invest the time, effort and, obviously, money in getting the most out of a very different type of bike. I’ve seen many triathletes spend a lot of money on a tri bike and end up less comfortable, less confident and slower.
The key thing to remember is that 80%+ of drag is created by the rider and not the bike, so to see significant gains from your new wheels, your position has to be better and you have to be able to ride in it.
A new position is going to take getting used to and there’s no doubt, in terms of feel and handling, that a tri bike is a very different beast to a road bike. Don’t buy it and then leave it hanging on the wall until race day, or only use it when you’re on the turbo. You need to put the miles in on it, race some time trials and work until riding it seems like second nature to you.