What should be the amount of change in the program to read the result is not a bugfix/extension opportunities, and other program?

The name change is sufficient.

For personal or marketing purposes can be considered as anything binarnoi the comparison showed hoe one difference, but in legal (definition of authorship, etc.) technically, even if one line with copyright law will find will be considered a violation. Actually, as far as I know, it's up to the examination without formal kriteriev, everything counts from the full match code fragments (including commentarii) to algorithms and architecture. Experts rukovodstvuyutsya the fact that no two people can think alike.

When radically changed the scenario — then changed the program. If the code has been rewritten twenty times, three times was done in the redesign of the interface, but the scenario remains unchanged is the same program as it did not want developers to say otherwise.

from MS is 3%
I believe that 100%.

Depends on the availability and characteristics of conscience.