29er fork compatible with a 27 wheel

Axle to crown will be higher. Therefore:

Stack will increase (bar height) Head tube angle will slacken, so will seat tube angle. Bottom Bracket height will raise.

A 27.5 Angel will lose it's wings

Will slightly alter the geo of your bike. Not a huge deal in my opinion. On a DH bike having a slightly slacker ht and higher bb isn't going to kill you.

But the higher BB may well change a lot. in terms of ride feel.

Assuming the axle to crown is only about an inch difference at most, we're looking at quite a small change in bb height, definitely not enough to ruin your ride. Playing around with your suspension would make a more noticeable difference.

In my experience the resale value of 29er wheels and forks is much higher, and they sell quicker if that's of any importance to you.

Go for it. The difference in bar height can be remedied by moving around headset spacers or handlebar rise, and the change in BB height is pretty minimal. And if you're upgrading to a fork with a significantly better damper and/or air spring, you will absolutely notice that on a DH bike.

Dual crown forks can can be adjusted up and down in the crowns to change axle to crown height, so it's worth measuring your current 650b fork to see if it's showing 203mm of stanchion between the seal and the crown. If it's showing more than that (lots of stanchion showing), you can reduce some of the difference in axle to crown when you install the 29er fork by setting it bang on 203. The axle to crown difference between two identical forks (one in 650b and the other in 29") is 19mm, so if you have an extra 10mm of stanchion showing on your current fork, you could cut that difference in half by lowering the crown to exactly 203 on a new 29er fork.

Adjusting the crown height is a great way to get things back to where they need to be. Another way to set the crown height as low as possible will be to let all of the air out and push the fork to full bottom. Then, slide the crowns down towards the dust whiper seals, leaving around 10-12 millimeters between the the lower crown and tire to avoid any contact under full bottom. Fox should have a specification for this in their owners manual but I believe they specify distance from the top of the stanchion to the lower crown instead.

Another thing to consider is what crown offset the new fork has vs what your current fork has. If steering feels weird after switching to the new fork and you arent able to get used to it, the beauty of a dual crown fork is the ability to swap the crowns out for a different offset. If this is the case, you can correct the issue for the fraction of what a fork costs. 

Both of these solutions are what allows Ohlins to make one set of lowers for every wheel size and selling different offset crowns for the appropriate wheel size. The method to set crown height makes me think other forks should be able to do the same since stanchion lengths arent usually specific to wheel size.