It would be pretty hard to screw up signing Lebron.
I don't see any way Doncic slips out of the top 3 and he should really go 1st overall. But if he or Jaren Jackson are there and that's all it takes to move up to #4, then it's an easy decision to trade up.
I'd have to think the Sixers would have a lot of competition in trading up to that spot. I would also guess that Memphis would want to unload one of their bad contracts in order to move up. Probably something like #4 and Parsons for #10, Covington, and Bayless.
It's a steep price to give up Covington, but is definitely worth it for Doncic and the Sixers would still have a lot of cap room to make an other move.