What must change when porting a classic ATmega project to a newer AVR family?

Compare the exact device datasheets for memory organization, peripheral registers, interrupt vectors, clock setup and pin multiplexing. Similar peripheral names do not imply identical configuration. Port one peripheral at a time and retain a small behavioral test for each, such as a measured timer output or a known ADC input.

Check the programming and debugging interface and the required target-voltage conditions. Different AVR generations can use different interfaces, so do not assume an older ISP wiring arrangement applies. Review reset and configuration settings carefully and preserve a documented recovery path during development.

Use the new peripherals when they solve a measured need, such as reducing interrupt load with event routing. First reproduce the required behavior, then optimize it. Record compiler and device-pack versions and check startup, sleep and fault recovery in addition to normal operation before calling the migration complete.