Hejda very likely could fall off, but that's by no means a for sure thing. It's certainly possible that he could still be effective at 37, and his style of play and smarts might lend to that. If he doesn't, they have other options to replace him, rather than NEEDING to do so this year in trade.
As for UFA defenseman, that's only been the case the last couple years. The UFA market has been dead because teams wanted to lock up their players to cover the new CBA, and the players and their agents wanted to do the same because of the uncertainty over the cap going down.
Starting in 2015, quality defenseman will finally start to hit the market. These are the guys that are scheduled. Many of these guys will get re-signed before hitting the market, but the list of guys scheduled is much more impressive than the last couple years.
The cap is about to go up significantly in the next two years, as opposed to only slightly, and then last year going down significantly. That will lead to less player re-signing with their teams, and more hitting the market There will be a few good options to replace Hejda if they are so inclined.
2015
Mike Green -not good defensively
Paul Martin - older, and will be re-signed
Z Michalek - will re-sign with Phoenix
Marc Staal - going to Carolina
Eric Brewer - will be 36
Brad Stuart - will be 35, will re-sign if still good
Francois Beauchemin - will be 35, will re-sign
Barrett Jackman - will be 34
Marc Methot - really a #3, but would be a solid signing
Andy Greene - older, and will be re-signed
Anrej Sekera - would be a solid signing, don't see Carolina letting him slip away for nothing. re-signed or traded and re-signed.
Jordan Leopold - been there
2016
Brent Seabrook - will be re-signed
Keith Yandle - will either be re-signed or traded and re-signed
Alex Goligoski - I shouldn't comment on him
Kevin Bieksa - will be 35
Dan Hamhuis - will be 34
Braden Coburn - would be a legitimate option if he made it to UFA
Marc Giordano - would be 33, but playing style should allow him 3-4 good years at least
Tim Gleason - barely a middle pairing player
Luke Schenn - a 3/4
Nicklas Grossman - middle pairing
Plus other RFA's that didn't sign long term previously.