What Jiang Yucheng meant was that he would be using the things which had been prepared for his wedding with the late Crown Princess. In fact, a lot of them would be excluded as the scale of a mere princess's wedding wasn't as huge as that of the Crown Princess.
Although Yuwen Wei had perfectly relayed Shangguan Wan's reluctance to use the things left behind by her half-sister lest she feel emotional when she saw them, Jiang Yucheng still decided to do things his way.
While Jiang Yucheng might seem like he was doing this out of consideration to relieve everyone's pressure from working on crunch time, the reality was that he didn't give a hoot about Shangguan Wan's feelings. If he were truly sincere about her, he definitely wouldn't have done things so carelessly because their wedding day was also the day of her ascension to the throne. It would be a spectacular event and the talk of the entire Tianling Dynasty.