I've got good news for everyone who is waiting for the upgrade system: it's nearing its completion.
Since the system is used for my campaign, it needs to be completed before I can use it on any map. Especially since the campaign has a new game +, even the first maps must support the unit group, upgrade and leftover feature. So I've been working on the past 2 weeks to finish the damn thing.
With great success: It's almost done, and once it's done I'll implement it on the maps and test it under real-life circumstances. Once it's 100% fested and bullutproof, I'll make a tutorial and example map for it. This will happen at the end of this month (20th-30th), or early September at the very latest.
I thank everyone for their support, it wouldn't have been possible without you (and some other threads I found here and there).