So in the last session my players killed the BBEG's general a lot earlier than intended (he was supposed to die in the final session, which is about 3-5 or so sessions away). We ended right after they killed him, so I was thinking of potentially making it so that they killed something that Shapeshifted into the general rather than the actual general. This would then create a small subplot of lots of creatures impersonating the BBEG and the general all around the country. (Was thinking like maybe a Gray Slaad or something, and just claim the BBEG found a way to create/summon them, as they are already summoning demons). This would help potentially help me back up what I have told the party about the BBEG terrorizing the whole country, cause he is technically everywhere and constantly causing chaos, but in reality he isnt.

Would this be a bad idea to do as it might take away from the party's achievement, or not?

TL;DR: Party killed major villain earlier than intended. Should I claim it was something that shapeshifted into the villain so I can use him again later?

EDIT: Thanks for the advice everyone. I am letting the party keep the kill, but am adding some Changelings to the villain groups grunts who will be acting like all the dead generals (party has killed a few others before that werent as big of a deal) in order to help terrorize the country. They obviously wont be the real deal, and now the BBEG has a new right-hand general who is gonna be extremely sadistic. Also with most generals gone, BBEG is gonna have to commence his plan earlier than planned. (A lot of this is from advice you guys said).