I have been able to swap all the Halloween decor EXCEPT for the large jack o’lanterns flanking the..

Also took me a while to figure out :-/ You have to tap on the greyed-out potted plant icon, directly above the jack o'lantern (and slightly below the similar icon for the potted plants) on the right to trade it out for topiaries.

You can't remove the Jack O' Lanterns, only replace them. You'll only be able to replace them after you reach the level in the 3rd section of the office area where Jia projects big topiary plant vases to bee put by the main entrance door (where you have the Lanterns). If you got the Christmas decoration in the Xmas event you can also replace the lanterns with those snow dog figures.