If your looking to change barrel length, you can do that with a few matching parts (bollt/barrel/gastube/gas block) and a gun Smith. Use the same brand upper. Or if you want a different hg it should * come with the proper barrel nut ( per upper thred spec) to fit the hg you desire.
Depending on what you can actually find and buy, some parts may interchange between the new and old parts. Talk to the person you choose to do the work for best results.
Doubtful a complete upper is available these days but I could be wrong.. happy hunting.