Though there are lots of disadvantages, there are also lots of advantages in upgrading your TH9 war base to TH10 war base. If your focus is more on war and not on farming, this would sound like a good plan. The main advantage that you see here is that your new TH10 war base cannot be 3 starred due to the defense of inferno towers.
By staying in TH9 war base, you will face hard time in 3 starring a TH10 war base. The match making may not look like even but after you upgrade your TH to level 10 you will get high matchmaking bracket. Another advantage associated in upgrading to TH10 is that you will be able to get max troops that will be really helpful during your war.
When you are transforming from TH9 war base to TH10 war base, it will not take lots of time for you to upgrade because you would have already maxed out all your buildings and it will take only less time for you to upgrade your buildings and make a strong TH10 war base. Though during the initial stages of upgrading you will find it difficult to donate troops, later you will be able to donate max troops to your friends.