I want to buy Dell XPS 15 9570 (NOT 2in1) with matte FHD screen - but it seems that at my country it will sells only with HDD + SDD + small battery 57Wh. Is it possible to later replace battery to bigger 97Wh? Will it fit? Will it work? How much it will cost?
It'll work - for about $150 for the battery. You'll also need to remove the 2.5" drive -- the larger battery takes the space used by the 2.5" drive.
Ejn63's answer has been accurate at least for the 9530, 9550, and 9560, and possibly older generations that I'm not familiar with, but if you're concerned it won't be possible for the 9570, then wait until support.dell.com has the Owner's Manual posted for the 9570 and then look at the diagrams to see whether the 2.5" drive continues to occupy the area that the larger battery would use. But I'm 98% certain it will, since the 9570 chassis design hasn't changed from the 9560, and looking at teardowns of the 9560, there isn't anywhere else Dell could put a 2.5" drive so that you could have both a larger battery and a 2.5" drive at the same time.
It is based on the 9560 -- no one outside of Dell has seen the specs on the 9570. The only thing I'd guess that's likely to be different is that Dell soldered in the RAM in the 9575 -- I'd bet the only real difference inside the 9570 will be soldered-in RAM vs. the 9560.
I want to do the opposite. I have the longer battery in the 9570 and want the shorter one so I can put in a hard drive. I don't suppose we can swap?
I can confirm, the 2.5 inch drive will need to be removed to hold the larger battery.
Additionally, the RAM is not soldered-in in the 9570