It's been a long time since I've used a C64, and I have never used that assembler. But... you could try saving your file as @:FOO (or possibly @0:FOO). I think that was the syntax for "save and replace".
You are awesome!! @: did the trick! Thank you so much!
I'm sure I'm missing something silly here, but surely I should be able to write to disk to the same file name once I've worked on more code in Turbo Macro Pro on the C64, yes? For example, I work for a night, then write my file to disk either as a sequence (via -> w) or as a binary PRG file (via ...