I completely understand the whole stopping the production of improvements your don't have the tech for. As after you concur a planet, the plans aren't going to be stored in a nice little box. However, already built improvements should not be destroyed, but be maybe halfed. Which makes sense as you would not have the running instructions. Then at least they are there, and the planet is not thrown back to the stone age, but you will probably want to build over them.
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I think this is the best idea in this post being that if one took over a planet, leaving the buildings in tact, they would still be there. So there fore even though I don't posess this technology I should be able to use it at least to some means. Adding to this though if there are a planet full of one specific building you should be able to figure out this technology or be given a greater chance (based on the number of these structures) to learn this technology so that if there was one Industrial Sector you probrably won't be able to figure it out but still be able to use it at some level weather it be 10% or 70% depending on your level of technology. On the other hand if there are 20 Industrial Sectors you should be able to almost be sure you figured out the technology, however, again based on your tech level.
Just my ideas, sorry if I lost you.