Is the I²C frequency meanwhile changeable? I read a few articles a while ago, that the frequency is not changeable.
Yes it is, for all the info available on the I2C interface (which is part of the BSC, Broadcom Serial Controller), I suggest reading Chapter 3 (BSC) of the BCM2835 ARM Peripherals datasheets.
On the first page of Chapter 3, it mentions the Clock Divider register. On page 33, it explains this register in more depth: