The difference in comfort between first and second class is not very big in Germany. Just a little more space and a German Newspaper. A first-Class ticket is roughly 60% more expensive than a second class.

Changig for just a part of your journey the class shouldn't be a problem (I just checked it for Interrail und EurRail passes). Which pass do you have/want to buy? The cost of the change would be the price difference between a regular second class and a regular first class.
cheers
757DUD