Almost positive that once terms of a trade have been submitted to the league office, the terms are final and complete. The only way the C's could change the terms of the trade would be to create another trade that changes the terms of the first trade.
So Boston and Philly would have to complete a trade where if the Lakers pick doesn't convey to Boston based on the terms of the original trade, the pick would convey to Boston no matter where the pick lands in exchange for....
What that exchange is would be important. My guess is it wouldn't be for just swapping the 2018 conveyance for the 2019 Sac. conveyance. Gotta remember, the Sixers could still end up with the 1st pick this year and the first pick next year. Gonna take a lot to have them give up that chance for the Cs to get the LA pick this year.