Did Brahms introduce any significant changes to the symphony?

Typically it's just the number, key, and opus number as Jim Paterson has pointed out. read more

It is often remarked that there is a strong resemblance between the main theme of the finale of Brahms' First Symphony and the main theme of the finale of Beethoven's Ninth Symphony. Also, Brahms uses the rhythm of the "fate" motto from the opening of Beethoven's Fifth Symphony. This rather annoyed Brahms; he felt that this amounted to accusations of plagiarism, whereas he saw his use of Beethoven's idiom in this symphony as an act of conscious homage. read more

Not really, no. It is in the form of the symphony that Brahms is at his most conservative, and so his symphonies are thoroughly Beethovenian and Schubertian. read more