I had imported a project into AS6 from AS4.
2 of the files in the project had not been used (GLCD driver files) for a while and I had done several changes to the current version files.
So I fixed them on another computer with AS4 and then copied the files into AS6, same directory overwriting the old files.
Spend quite some time trying to recompile the project but Studio kept on insisting that the new or renamed functions did not exist even though I could see them inside the new files which were now part of the "solution" (I hate that word).
The only way to fix things was just to delete everything inside the project folder and re-import the project into the same folder, then I only had to add a single const to some data in flash.