I can't comment for sure if they still have the source, but I know that EA made changes to those games when they released The First Decade [0] to make their CD-checks accept the new DVD. Most of the games were recompiled (so source was available) for that.
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But the RA2: Yuri's Revenge binary was just patched where needed. This distinction was made because at the time YR already had some hardcore fans decompiling it and enhancing it, EA's community manager heard about this and made sure that community effort was not destroyed. I was one of those fans, and taking it apart was one hell of an adventure. We didn't decompile all of it, but we figured out many details, and added a lot of bugfixes and enhancements. It's not the same thing as the real source code, but you can see what we got in [1]/[2]/[3] (unfortunately those haven't been updated in a while).
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But the RA2: Yuri's Revenge binary was just patched where needed. This distinction was made because at the time YR already had some hardcore fans decompiling it and enhancing it, EA's community manager heard about this and made sure that community effort was not destroyed. I was one of those fans, and taking it apart was one hell of an adventure. We didn't decompile all of it, but we figured out many details, and added a lot of bugfixes and enhancements. It's not the same thing as the real source code, but you can see what we got in [1]/[2]/[3] (unfortunately those haven't been updated in a while).
[0]: https://en.wikipedia.org/wiki/Command_%26_Conquer:_The_First...
[1]: https://github.com/Ares-Developers/YRpp
[2]: https://github.com/Ares-Developers/Ares
[3]: https://www.modenc.renegadeprojects.com/Contributing_to_Ares