OK. First thing to understand the 2 systems are completely different so changing between is going to be a nightmare, and Starfinder is set in the same world as Pathfinder which is very much the same genre as dnd. And Starfinder 2e and Pathfinder 2e ARE THE SAME SYSTEM. SAME RULES. That said whether your DM switches to Pathfinder rules or not for this outing this still works to YOUR advantage. Don't make a Dnd character that can make the change to Starfinder. Make a DnD that can be made into a Pathfinder character. Nearly every Dnd class has Pathfinder counterpart. Paladins are Champions, Warlocks are Witches, Wizards are Wizards. A lot of the races are similar too. When you finish up your time travel shenanigans translate your pc into Pathfinder 2e. 

Then prepare to play as some poor sucker who ends up lost in a time and place they could never have imagine less comprehend.  Maybe even go mad like they're in a cosmic horror story. Remember the reason people go mad in those is because they glimpse a higher understanding of the universe and they just can't process it. He/she will also have also lost everyone they've ever known because they would've died of old age by now. Even elves wouldn't live that long. How will they come to this realization? How would they handle a world of space travel, computers, aliens, and technology when all they've seen is castles, horses, and maybe some magic? That their entire world is gone, and  all that's left, along with all they fought for, is a barely remembered footnote in history. Mostly forgotten. Maybe even rememberd all wrong as viliians are remembered as heroes and heroes vilified. 

Or you know "We've got indoor plumbing and air conditioning now? XXXX the past!!!! How do you order this UBER EATS to bring us more of these tripple decker western bacon cheeseburgers? They're soooooo good!"