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The transition to 100 percent Unicode support in Leopard was undoubtedly the biggest change to occur in the AppleScript language since it was first released in 1993. Sure, the implementation of AppleScript may have changed a lot since then, moving from 68K processors to PPC and then Intel, jumping from Mac OS 9 to Mac OS X, and growing 64-bit support along the way.
None of these behind-the-scenes changes really affected how scripters interacted with AppleScript, though: scripts written even on early versions of AppleScript continued to work, with perhaps just an occasional quick recompile needed.