The biggest was that the FUSE and FUSEX register bit assignments changed. This requires reassembly of the original source (with some changes to the assembler directives) in some tools, or with the SX-Key, use of an entirely different executable. The newer devices are obviously supported by default in all SX tools. You can also load hex files designed for older SX in the SX-Key, and then manually choose the device configuration in the device window before programming. The most seamless approach (if you have the source) is to simply reassemble with the right tool.
Other changes in the SX from older datecodes included, reduced number of options for internal RC oscillator frequencies, low voltage support (2.5 - 5.5V vs. 4.5 - 6.25V), expanded oscillator gain settings (more reliable crystal operation) and various bug fixes.
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