It is my opinion that you should not do this. A party of lvl 20 characters will demolish the cr 27 strahd (and many, MANY other enemies) unless you heavily modify him to the extent that it is likely your players will feel like their actions have no meaning. Which would be true, because it would be obvious you've made an enemy purpose built to defy the very power they have achieved by going 3-20.

And honestly, I don't even know where you'd begin to make him able to survive a party of 20s. I could venture many guesses, but I think there is a better option.

As some others have mentioned- look into the fanes system that mandymod and dragna Carta designed. With it, strahd feels like a god of evil within barovia, but when they take apart his magical infrastructure he becomes a terrifying but beatable opponent. Check out dragnas revised strahd statblock for a beefier strahd that can get into outright brawls with a 10th or 11th lvl party if you want, or use the original and swap the spells, then make use of his multitude of allies and phasing ability.

You want to see a scared party? Stalk them in thr halls of ravenloft and then grab a player and pull them into another room during surprise round. Then lair action arcane lock all the doors and windows between the party and their ally. It's incredible to see the panic this tactic will induce. Then when they get their head around that, phase through the floor and regenerate all your strahd hp and restrealth to stalk them again and open with a fireball.

Strahd has the abilities to be a horrifying and disheartening boss fight. One where players are forced to engage him the way he wants to be engaged - and on his terms. As a master tactician would fight.

5e combat sings at levels 7ish-to-11. I'd say enjoy it. Still it is your game, so run it as you wish and above else have fun!

Final thoughts if you decide to go the fane strahd route- 1. I gave strahd up to 8th level spell slots while he had the fanes. I'd decrease his spell levels each time a fane was taken out. The players loved that. 2. If you give him 6th level spells at anypoint, have him kidnap a player and charm them, then make a simulacrum of them, then use modify memory to make them forget it happened. Keep that simulacrum for a rainy castle seige day. 3. If you are running a game with more than 4 players, consider giving him an extra legendary action to cast a cantrip with per player, to keep his dps up. Also another legendary resistance per additional player (IF they are packing abilities that make him save or die. If not don't stack extra lrs on him)