Question about West-Slavia

Brand new ck3 player. My first real campaign im playing right now us polania in 867. I was initially going to unite the west slavs, but Poland formed early and really strong. I instead made Pomerania and formed a super cursed polish slovanskian baltic empire. Now I'm strong enough to go unite the west slavs, but if I do so while I already hold an empire will that cause succession problems or will it destroy my current empire?