Joselu will most likely lose his position as a striker because it seems like Mbappe is being prepared to fill that room. When Mbappe arrived, it was clearly impossible to create a situation where Mbappe and Vinicius Jr were fighting for position, and it was also impossible to shift Mbappe to the wing-back position. So the most feasible thing is to play Mbappe in the center of the front with a 4-3-3 formation which is also often played by Real Madrid.
It's nothing new for Real Madrid to play out that scenario because in the past Real Madrid once placed one of its world-class right players in the central midfielder position. At that time Real Madrid already had Figo who was always the main star on the right wing. So when Real Madrid signed Beckham, he had to be willing to play in the midfield, and it turns out he is quite good at playing in that position.
Joselu is no longer young and even this season he didn't get a main place so when Mbappe comes later it's clear he will be left out, but I think Joselu is still needed by the team, to be a cover on the front line.
For the front line, I think Vinicius Jr and Mbappe can still be a partnership or play on the wing for one of them and indeed if you look at the quality I think anyone between these two players can play on the wing or in the middle and what is certain is Vinicius Jr and Mbappe's attacking instincts strong enough that I don't doubt them when playing up front.