Against my wishes and better judgment,I have had to change travel plans and tickets from March to the last week in July and the first week in August. Thinking the mountains will be cooler, we are concentrating on Bavaria and the Salzburg area, using trains the first few days and then a rental car over two weeks,covering Regensburg(first day), Munich(four days), Ludwig's castles(two days), Berchtesgaden(four days), Salzburg(two days) and Erding or Freising (final day). Our schedule is flexible, however. My biggest concerns are summer heat, crowds and higher prices, none of which have been a problem in the spring. I would welcome any tips any of you would like to share for avoiding or overcoming these problems and making the most of this trip.
Concentrating on the Alps at that time of year is about the best you can do to avoid hot temperatures. In the few years since I moved over here, though, I've found German summers rather comfortable, and this is coming from someone who absolutely HATES temperatures above 30° C. But, there can be short heat waves, and even these can hit the Alpine regions. At least the mountains will mitigate the heat a little. I generally find the fear of summer "crowds" in Germany and Austria to be overblown. Unlike the popular museums in Paris and Italy, German and Austria have few sites that really pack people in and require long waits to enter. On your list, Regensburg, Erding and Freising will not seem crowded at all, Munich will feel like a big city at any time of the year, and Berchtesgaden will have a lot of visitors, but nowhere near it's winter time capacity. Some areas of Salzburg will feel over-run with your fellow visitors. And of course, expect a mass of humanity gathered around the Ludwig's castles. However, if you pre-purchase your tickets, the crowding won't be an issue, because your ticket buys you entrance at a set time.