I'd do the opposite and have the drows move closer to the entrance of Undermountain. I dont know if there is a nice ambush spot in the first few levels? Or have them somewhere in Skullport, going for a more Urban encounter.
The way level 3 is presented, there's a bit of a "cold war" between T'rissa's drow forces, who have captured the eastern half of the ruins of Stromkuhldur (an old dwarven town), and the goblinoids of the Legion of Azrok, who still control the western half. The Legion is all that stands between the drow and Skullport, and the Xanathar Guild is busy infiltrating the legion via bodysnatching (their mind flayer ambassador is slowly implanting intellect devourers in more and more goblins' skulls).
The book states that if the drow are defeated, the legion retakes the eastern half of Stromkuhldur, but their victory is short-lived, as the mind flayer eventually succeeds in bodysnatching Azrok himself and his loyal wife, Lurkana. With the legion completely under the Xanathar's control, they act as the first line of defense for Skullport.
Now, the previous group of PCs did not
completely defeat the drow on level 3, but they did leave T'rissa with drastically depleted forces. I would think that the Legion of Azrok would take advantage of that fact and press the attack. They might not have much more luck than the PCs did, although they do have the advantage of numbers. Perhaps the Legion hired a different group of adventurers to help drive out the drow.
My main thinking here is to delay the gratification of my players, since their new PCs seem to be built around getting revenge / finding the previous PCs. I don't want that to be the first thing that happens. So if I have T'rissa withdraw to level 4 to join forces with her sister, Melith, and have Halaster steal away with the (bodies of the) previous PCs to lure the new ones deeper into Undermountain, that should work to extend things a bit.
EDIT: Actually, I might even make it so T'rissa withdrew to join her sister on level 4 but was then summoned down to level 10 to be interrogated by her mother. So the new PCs will have to wait a while if they want to fight and kill T'rissa specifically.
As for where Halaster puts the previous PCs - I might just determine that randomly.