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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.
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[i]in and around 12.5m lower[/i]
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.