Generally speaking, I imagine most users who wish to "Upgrade" their Comm-Unit would simply sell their old Unit for the 10% Return and simply purchase a New, better model.
I imagine it would be a pain to find some NPC craftsman or merchant who'd even be willing to tear a pre-build Comm-Unit/Computer apart to upgrade it, especially since it would be much faster and convenient to buy the new one and recycle the old model.
If you're looking at allowing a PC to upgrade the unit itself, I'd simply work out the math difference between the two device that they'll have to spend in UPB (Or other Crafting Materials) and purchase the higher tier equipment to replace the previous version. Once you have the upgraded version added for the appropriate cost go ahead and sell the old Comm-Unit for 0 credits.
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Ryan Furnas
Lone Wolf Development Support -
support@wolflair.com
http://www.wolflair.com