You are allowed to add necessary features or port the software as well. Of course, what counts as a "necessary feature" is always up to debate, but if you for example own a software that only runs on Microsoft DOS and you want to port it to Windows 10, that's allowed.
Also, as far as copyright/patents are concerned those aren't a Problem with reverse engineering. If you reverse engineering something and you then discover copyrighted code while fixing a bug that's not a problem. Knowing of that code is not copying it.
If you were to copy that code or use knowledge you gained by reverse engineering to build your own product that would be an issue though.
I dont think I've ever sold software, just a licence to use said software, so I think my old dos programs will be safe.
> If you were to copy that code or use knowledge you gained by reverse engineering to build your own product that would be an issue though.
And thats the hard part, how do you prove a thought process existed before you had your thought confirmed by seeing it used in some code that you are reverse engineering? Even if I had written such thought down, proving the date & time on the piece of paper isnt back dated is also tricky.
Also, as far as copyright/patents are concerned those aren't a Problem with reverse engineering. If you reverse engineering something and you then discover copyrighted code while fixing a bug that's not a problem. Knowing of that code is not copying it.
If you were to copy that code or use knowledge you gained by reverse engineering to build your own product that would be an issue though.