Don't do this. If you want to run a 5e vanilla campaign, just do that. If you do the basic Quantum Leap idea, do it without pregens. End of the session, with the time warp or whatever, and tell people to come back with new sheets, "an ancestor of your current character. Level [N]."
Depending on the length of time player are expected to play them, Pregens are OK. If its just like one or two sessions, like the Pregens have a quick scene to play, its fine. Any longer than than that... well, you
really need to rework your narrative. But If you won't do that, then the players should be building (or at least having major design input) into the characters they'll be stuck with for a month or more.
I've had players play pregens for 1-3 sessions (or 1 fight) backstory sections. Depending on your party it works.
For example, they had a very straight forward "Save the princess" plot, and as a fast way of getting the players upto speed on how the royal court worked. Instead of a whole protocol sheet and NPC descriptions/relationships, the players played a group of royal guards the night the princess was abducted. I framed it as "Your contact starts to tell you the story of how the Princess was abducted. He turns to you, hand gripping his goblet and begins 'It happened two nights ago, when the moon was full. It was a night like any other, and none of the guards on duty had seen anything out of the ordinary..." then I had the players roll a d6, and passed out new (simplified) character sheets (ie no inventory, only listed score mods, etc) for one of the guards and told them very quickly who they were going to be playing for the next session (which dragged out to two because my players are slow)
If the players tried to do something a guard wouldn't do, I'd just tell them "Your training as a royal guard stops you from raising your hand and calling out 'What's up my, Nigga?' as the king walks by", and usually just roll on (which I dislike doing , but as a one session, [which my slow ass players dragged to two sessions] putting them on some sticky rails is fine)
They were of course railroaded to failure, but the players did very good as guards. One of them managed to beat their death saving throws and and another actually survived, so those guards were available for questioning by the party when they arrived at the royal court. One of the players managed to land a crit on the BB's Captain who was running the abduction, so when the players fought the captain he had a bloody bandage on and was missing hitpoints because he was still injured.
Player reaction varied. Players were initially so-so on having to swap out for guards, one of them sort of bitching a lot. They accepted it when I told them it'd just be a temporary thing, and once they shut up and went with it, they were pretty into being guards by the time the big fight started, though the fight with the abductors... they knew the princess was going to get carried off so they thought nothing they did mattered at all and so there was player frustration and especially as things started to go wrong.
But then when the party got to the royal court the players
stood up an clapped really, really liked it when the King brought out the surviving guards for the party to question and that realized having living witnesses gave them intel on who they were going after and the players who had played the guards had fun talking to their alternative selves (I'd made sure to take note of anything the players did to add personality to their temporary characters and mirror it when I RPed them) or when they saw the bloody arm of the guy who lead the raiders because they realized that their actions
had mattered. Like really good player engagement when those happened.
OTOH I've also had that just not work, usually because the players just refuse to adapt to their new characters. In which case nothing to do than just wrap it up quickly.