There is one well-tested recipe that uses milk with lemon juice (or vinegar) instead: to begin, one has to heat the milk up to 36C-38C (body temperature), add 1tbsp of vinegar, let it rest for 15 min. I am guessing this is done to curdle the milk, right? Will this process work as expected if the milk is ultrapasterized?
It will not curdle as well as regular pasteurized milk will, but probably good enough for the recipe you're making.
Ultrapasteurized milk, because of its "cooked" nature, doesn't form curds as well as other kinds of milk. Particularly, you cannot make cheese from it because the proteins have been changed by the high-heat sterilization. They have been denatured.
However, this recipe does not seem to depend very much on the texture of the curds made during the curdling process in the first two steps. If you were draining the whey, I'd say not to use UHT milk, but in this case everything is still mixed in. At worst, the pancakes would have slightly inferior texture to those made with regular milk.
Yes, it will curdle. It is a chemical process where the proteins clump, the ultrapasteurization doesn't prevent it.
The other question is, do you need to do that at all. The whole reason to use cultured milk products in this kind of pancake-like recipes is that you need acid to react with the baking soda. So you can simply throw everything together, without premixing the vinegar and the milk. It will work quite the same way (unless you happen to pour it such that the vinegar and baking soda react too early). If you insist on mixing the milk and vinegar first, it would have worked if it didn't curdle (or it will work if you pick something which won't curdle, such as almond milk). Or you can simply switch to a recipe with baking powder and use whichever liquid (dairy or not) you wish.