Having the PCs fight their own characters could get tricky. I would bring Trex in as soon as they realize who they are fighting, that way the player's don't feel forced to kill off their old characters.

Also, have you come up with a reason why the new characters are fighting the old one? It appears that they were working for Teemo, was this something they you connected to using their obligation?

Once you work out the lumps, it should be a nice little transition.

Hm... That's nice, but it would be painful to play the game and have to watch as your old characters die. I'd maybe suggest a slightly more diplomatic approach where the created characters defeat the pre-gens but have them live. Maybe they knock the pre-gens down, and the pre-gens ask why they're being attacked. When the created characters say that Teemo hired them, the pre-gens reveal that Teemo is dead. Or at least something along those lines.

The only problem I see is that this combat is likely to go off your rails pretty quickly. You can't guarantee what the rolls are going to be and too much is riding on having a pretty specific outcome.

I'd suggest narrating the transition with your players input as to how thing turn out, it is after all a collaborative system.

I do need to work out the specific reasons for the new characters to be there, but one of the new characters is an outlaw tech that was a hermit on Ryloth. A part of his obligation is that he created assassin droids to hunt himself down; partly because he's too much of a coward to commit suicide but also to keep himself alert. So he may have overheard stories about the group being on Ryloth and made assumptions 41-Vex was one of the assassin droids he built (Vex delivered a precision shot to murder Angu Droomb). Also, one of the other new characters is a bounty hunter assassin, so he likely saw the bounty transmission from Teemo. Seeing that 50,000 credits is a lot, and being in the good graces of a Hutt is never a bad thing, it shouldn't be hard to justify them hunting down the pre-gens.

My group isn't particularly tied to their pre-gens and imagine once they swap sides they will potentially be eager to murder them anyway (especially if they are carrying lots of credits...). However, I will modify the pre-gen behaviors to match the group once they swap. Either way, if they try to kill them or not, T'rex will interfere and foil their plans. The players actions will determine what T'rex does; if they are blood thirsty and try to kill them then T'rex will decimate them instead (and get away with the loot), if they try to be diplomatic and let them live; then T'rex will arrive with an entourage and kidnap the pre-gens instead.

The only problem I see is that this combat is likely to go off your rails pretty quickly. You can't guarantee what the rolls are going to be and too much is riding on having a pretty specific outcome.

I'd suggest narrating the transition with your players input as to how thing turn out, it is after all a collaborative system.

I agree that it could go off the rails quickly depending on the rolls; but that's why I'll only give it like a round or two of combat. I'll be very cautious to not allow them to harm themselves too much before revealing the swap and leaving it up to them on how to proceed.

T'Rex will be introduced in a more narrative way, to avoid relying on rolls. He'll capture/kill the pre-gens and set himself up to becoming a nemesis for the new group.

This doesn't need to be some elaborate long lasting encounter. Just a fun way to tie the two groups together narratively before beginning a full campaign

Ah. I remember seeing the Hermit's character concept on a previous post. Here's hoping he doesn't read this.

For some reason I am just picturing Trex show up and all the pre-gen characters rip their fake faces off to reveal they are all actually Trandoshans, ala the -V- miniseries.