We'd need more information on what you changed to comment. Could it possibly involve a string delimited by apostrophes and you added an apostrophe in the middle?
var myString:String = 'This string doesn't terminate correctly.'; // error, the apostrophe in doesn't causes an error
This would cause a syntax error before compiling the SWF, but the idea is the same with any externally loaded text, such as XML. Characters thrown in can cause issues if they're not encoded correctly. Don't consider this example too literal considering the compiler would catch it.
If something simple like that is not the case you'll have to share the code so we can spot errors.
I decided to start from the beginning. I pulled the original files from archive, made the update to the .as file, republished .swf file and it's working. Mystery to me!
Thank you for replying!
Glad it's working! Mark any helpful/correct to close the thread so we can filter unanswered. Good luck!