There are differences in the mnemonics and other language components, but if you already know some assembly language of any other variety, you have already passed the first major hurdle in how to program at a low level by using registers and difference data addressing models.

There are some differences outside of language and mnemonic structure between the microprocessors that you have experience with and the AVR based microcontrollers. As to the ease of transitioning between the microprocessors and microcontrollers, that is going to be down to your experience.

Some areas will be familiar, but I would recommend studying the language reference and the AVR Instruction set manual to personally gauge this and to experiment with microcontrollers to gain more experience for programming for I/O operations.
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