I am unable to use upgrade items to improve double chests in-world, but picking up the blocks with the packing tape item lets me craft it into a double chest of the appropriate tier to place back down.

The problem with this is that crafting deletes a lot of the double-block's contents. As shown, a double copper chest getting upgraded to iron deletes 36 slots of content, almost half of the content. (an item is in each slot)

This is the only way I know to improve double chest tier/size without breaking the original double chest

This is in FTB Evolution, using version 1.2.8

ok, closing this given that and the fact I can't recreate. In case it happens to you with tier upgrades that have a correct "from" type feel free to open a new issue for that (as this one was primarily for the loss of items anyway)

So this is something I didn't pay enough attention to before because it only happens when a config is set to automatically pack storages.

I have fixed it so that items don't get lost when upgrading, but also added special upgrade recipes for double chests because you are supposed to use double the amount of ingredients with these (ingots / diamonds).

This is fixed in the latest release.

As for upgrading double chests in place - when you tried to do so did you have just a single tier upgrade in hand or two of them? double chest requires two because you are upgrading two chests.

seeing that this is a copper double chest and that it works for me in a different pack maybe just one more question - did you have copper to iron tier upgrade or just regular basic to iron tier upgrade? Because the basic one wouldn't work as it requires just basic chest

I had 4 in hand, as I was planning to upgrade two double chests. 2 also did not work.

...you know, I think this is one of those legitimate bugs discovered through user error. I almost definitely was using Basic->Iron/Gold since those were gotten as quest rewards, whoops!