The simple command is:

  sv@next migrate sveltekit-3

I'm upgrading hundreds of projects right now and it's going great so far. Here's the shell script I'm using; adjust as you like.

  find ~/git -path '*/.*' -prune \
  -o -path '\*/node_modules' -prune \
  -o -type f -name 'svelte.config.js' -exec dirname {} \; |
  while read -r x; do
    cd "$x"
    echo "y" | 
    pnpx sv@next migrate sveltekit-3 \
    --tasks all \
    --confirm \
    --install pnpm < /dev/null
  done