Cut pork shoulder into approximately 2-inch cubes. Set aside.
Put chiles in large heatproof bowl. Cover with boiling water, and soak for 30 minutes. Add chiles to blender, reserving soaking water.
In sauté pan over medium-high heat, warm lard (or oil). Add onion and cook, stirring occasionally, until charred in spots, about 5 minutes. Reduce heat to low. Stir in garlic, oregano, cumin, cinnamon, sugar, vinegar and salt. Cook another 2 minutes. Add ¼ cup chile water to deglaze pan. Add pan contents to blender.
Purée until very smooth. Use just enough soaking water to easily turn blender blades.
In large bowl, toss pork with sauce. Cover and refrigerate for at least 12 hours, and up to 2 days.
When ready to cook, heat oven to 350°F. Transfer meat and sauce to covered ovenproof pot or Dutch oven. Bring to boil over medium heat. Stir, scraping up any sticky bits from bottom. Cover tightly, and place in oven. Cook until pork falls apart when prodded with fork, at least 2½ hours.
Break up pork chunks to soak up more sauce. Sauce will thicken slightly off heat. If liquid is too thin, remove pork with a slotted spoon and reduce sauce over medium heat. Salt to taste, if necessary. Serves 8.