So many other good tips in this thread. I just finished CoS. Some changes / tips:
Baba Lysaga ("BL") is too cool and has too cool of a back story to remain a side character in the swamp. My party never went to the Ruins of Berez, but through luck and smart game play they assembled a powerful group of followers to help them storm the castle. Now, remember, BL thinks of Strahd as her son. She has been working to counter the work of the Were Ravens. She has spies and scrying and the druids on her side. Her influence reaches FAR beyond Berez. The party saved the Wizard of Wines and they defeated the Druids and destroyed the Gulthias tree on Yester Hill. So she KNOWS about them even though they didn't know about her. So when the party was ready to storm the castle, I had her ambush them and managed to at least kill a few of their followers and thin their ranks. In hind-sight, I would have made it even more of a challenge by having her appear with a good number of scare crows and druids. As powerful as she it, once characters get to tier two, they can take her on.
I increased Strahd's Power using the version in
Elven Tower's Curse of Strahd DM's Guide as a base with some modification. First, Strahd was a great warrior in life, why wouldn't he be able to swing a sword and wear armor. The animated armor that travels around the roofs? Well I had is so that Strahd could summon the armor and a blade as a lair action. The blade is a corrupted moon blade. Basically, the back story is that the blade was relic of the dusk elves and that it was corrupted by Strahd, who used it to kill the dusk elf women. This greatly increased his power to make him more of a challenge for 9th level characters played by experienced players.
Changing some of his spells is another way to increase the challenge. Giving him telekinesis is particularly useful. Srahd successfully used the spell to rip the Sun Blade from a characters hand and throw it out a window and 1000' down the mountain.
For the Night Hags in Bone Grinder - I gave them the full coven treatment with all the abilities from Volo's Guide to Monsters. The party's first run in with them was almost a TPK but instead of killing all of them, I had the hags take Irena to deliver to Strahd. I also had them forcefeed their pastries to all the party members to get them addicted to them. Finally, their night mare powers continued to weaken the party requiring protective measures until they were able to get the Abbot to help release them from being preyed on by the night hags.
I made the Exethanter (Ch. 13, X27) the architect (Ch. 13, X20), and Khazan (Ch 11) the same person and changed the background a bit. Not only does it make more sense this way that the architects room is in the Amber temple, but it also give more opportunities to get hints about the Lich and his background. In particular it gives opportunities for the party to learn his name and perhaps use it to get by the arcane lock to his phylactery.
One thing I would do differnetly. Only one member of the party should be able to claim any dark gift. Two players accepted the dark gift of Zhudun, the Corpse Star (Ch. 13, X42). Now one would think that this just means that the party can resurrect dead party members twice. But instead:
* After they killed the Death Slaad in X33e, they took the control gem from slaad corpse brain and then resurrected it, creating a Death Slaad minion.
* The second resurrection was used on the skull of Argynvost in K65. Giving them a Silver Dragon ally.
I could have easily prevented either -- but it was fun, so I went with it.
Another thing I would have done differently, is had Strahd invite them earlier. So they would have to fight for their lives to get out of the castle. They keep avoiding the Castle and were 9th level with followers and the sun Blade. Most of the dangers in the castle, other than Strahd himself, were not much of a threat to them.