If the new backpack is on your character and the old one is on the ground you can double-click each item to fast place in your inventory. Placement is random and dependent on where space is available.

would be OK if you could define where to place the items (e.g. put items first in my new backpack, but it seems backpack is always the last one filled)

this way the items are in random clothes and you have to sort it manual anyway :-(

it would be cool if the mechanic prioritized the item in your hands as a way to select which clothing slot is the primary.
Say drop full bag, put new empty bag in hands. quick loot the full bag, and the items to into the hand item first then randomly to others when full.

or just saving the sequence of the clothing (e.g. my backpack is placed as first item)
what i have placed on top fill first, than seconds…

would also be nice if the game didn't insist on putting matches in cooking pots full of water

I'm not a Dev, however I would assume the simplest way to do this is that if the UI is not open on the piece of clothing the new items will not be placed in there.
For example you can open or close the inventory of each piece of clothing or backpack while in the "inventory menu", if you hide those from view the item would be placed in the next available location that is visible while you are in your inventory. It's a new system that would require some getting used to however the player has the control and can learn how it works to prevent issues.
Also set a priority, container in hands gets first item placement choice then vest-coat-belt-pants-backpack. If any are hidden from view in UI they are skipped and cooking pots, drybag, frying pan, med pouch ect can only be filled if they are in your hands.
Doing this should result in items not being placed in hidden spaces when you just take it from the loot location. Only drawback I see is it forces the player to place the other container mentioned above in their hands in order to pick up any items that no longer fit in the clothing/backup.

Worth testing it out in my opinion @Geez