Whats the worst thing that can happen using 26″ wheels in frame that`s made for 27.5″. Can I use now and upgrade later?

You’ll smack your pedals on the ground more often. So yes, you can.

Bottom bracket will be low – in and around 12.5m lower, so depending on your frame and cranks, you might find yourseld smacking pedals on rocks, etc. a lot more than you would otherwise.

Aside from that, you’re good.

in and around 12.5m lower

huge if true!

The offset of the fork would be out too. If my engineering brain is working this early then it would have a similar affect to steepening the head angle a bit to the steering but not much else.