Changing from HR to Power what do I need?

you need a power meter.  

since you probably like your FLOs- then don't get a 'hub' based Powertap.  You can choose between a Crank, Chainring or Pedal based power meter.  If I were shopping today, and not wanting to spend a ton of money but wanted a good power meter- I'd probably look at the Power2Max chainring based units- as they're lowering their price.  If I were switching bikes a lot, and wanted to be able to use my FLOs on a road and tri bike, then I'd probably spring for the Pedal based system.  Powertap's P1 is the slickest.

And then....

 you want some way to start interpreting the data.  Training Peaks, Golden Cheetah, Strava Premium- or I just found "Endurance Tracker" which is free and seems fairly slick.

Your 910xt is totally power meter capable. you don't "NEED" anything else as far as instruments go. However, it's kinda' nice to have a dedicated bike computer mounted right in front of you, rather than on your wrist.  you may want to pick up a garmin Edge500 as a great basic computer, or a 510, 520.  all good.  The Polar m450 is cheaper and will pick up power, but I'm not sure it calculates the Np, IF, RSS values (you'll learn about them) that you might want.  My only complaint with the Edge 500 is that the GPS lock is kinda' slow.  Sometimes I wait for a minute or two.   and the navigation is limited to a breadcrumb trail and the firmware is kinda' iffy on that.  the breadcrumb blinks out sometimes.

A power meter will talk to your 910. So no need for the 510... unless you just want it

If you race the Flo's and train on other wheels, a crank-based or pedal-based system will make your life pretty easy, unless you want to purchase two hub-based systems, which isn't as expensive as you might think.

DC Rainmaker, as always, is an excellent resource: http://www.dcrainmaker.com/2014/09/buyers-guide2014-edition.html for reviews, and he has a product comparison chart as well: http://www.dcrainmaker.com/product-comparison-calculator?type=power-meter#select 

With Interbike happening in September, and Eurobike having just wrapped up, there's a LOT of new stuff coming down the pipe, too - and price drops on existing stuff!

That said, I run a PowerTap G3 (hub-based) and use the Joule computer... I don't like the Joule, but it came with my PowerTap (which i love) and I don't have another device that's set up for power - I use a Garmin 220.

I have the same wheels and watch.  I ended up getting an Edge 510 because I wanted it for the extra screens and I like it in front of me.  I bought a Power2Max Classic.   Best investment I could ever make.  I trained by HR and now train by power on the bike.   I still keep an eye on the HR because it tells the rest of the story.  A watt is watt no matter the variables that influence the HR.