Can new iMac output video to current generation Thunderbolt display as secondary monitor?
Your dual monitor setup will still work fine, and they will both stay at their native resolution.

The only issue is that you might have the borders will not match up, since 5k is (probably) a lot more than what you had before, so it will be a lot larger. When mousing from your old monitor to your new monitor, you will have no problems, but when mousing back you might hit a boundary on the top or bottom. You can configure where the boundary is, so it shouldn't be a huge problem.