OK, things are a bit different for a Half Dome 2+ than the older Half Dome, so having had to replace the shock cord on 2 tents, figured I'd share. It really isn't too bad, and seeing as we broke a tent pole segment because the old, loose shock cord was in the way, you might want to do this as soon as you notice slack in your shock cords.
First off, you can use the 1/8" shock cord that REI sells (usually out of their shop, if they have one, or online). But there is 3/32" shock cord sold from Coglens (here, it is at an Ace Hardware); it can make things a touch easier. I had the 1/8" shock cord and figure it is a bit heavier so maybe will last longer.
On the Half Dome 2+, the shock cord in the four legs of the tent poles run from the screw-in peg that fits the grommets on the tent up to one of the two four-way plastic hubs. A fifth shock cord runs from the screw-in peg one end of one of the ridgeline poles through near plastic hub and through the middle ridgeline pole through the other hub and on to the screw-in end of the other ridgeline pole. I'll focus on a leg of the tent pole.
There are only two simple tricks. One is that the grommet-fitting-end of the pole unscrews and is a hook holding a loop of the shock cord. The other is that there is a small plastic clasp in the plastic hub that you can push back out when changing the cord. Greater detail is below.
Tools: needlenose pliers, small vise grip pliers, and something to cut the shock cord with. Do not precut the new shock cord! We'll cut it at the very end.
Lay out the pole frame; I found it easiest if I kept the other three legs folded up and banded while working on the other leg. Go to the tip and unscrew it, revealing the end of the shock cord.
Unhook the shock cord.
(If you wish, at this point you could just pull the existing shock cord all the way back to the plastic hub and then thread your new shock cord in from the tip end and skip down a ways. I chose to thread the shock cord up the tent pole segments with the old shock cord to avoid accidentally missing a segment or putting a pole in backwards).
I looped the new shock cord through the loop at the end of the old shock cord:
I would then pull up the new shock cord by pulling the old cord up at the upper end of the tent pole segment. It can take a bit of a tug (which is why I did this segment by segment). Repeat through the tent pole segments.
(Pick up here if you just slid the new shock cord through the segments).
Now we have the new shock cord through all the tent pole segments and need to attach it to the plastic hub. Make a simple knot in the shock cord about 5-6" from the end of the cord. This will anchor the shock cord and it seems to work best if the knot is on the main length of shock cord rather than the small end piece.
Now we need to get the old shock cord out of the plastic hub, and more importantly recover the small plastic latch. To do this, we need the ridgeline poles out of the way. I folded the end pole back under the other poles and then pulled the shock cord from the middle ridgeline pole through and anchored it with the visegrip. This will allow us to pop the latch out.
Usually all you need to do is to grasp the two old shock cords and push them into the plastic hub to have the plastic latch pop out in the middle of the hub. But if necessary, you might have to reach in with the needlenose pliers.
The latch will snap open to release the cord (but be gentle, you don't want to break this piece).You now need a loop of the new shock cord to get through. Pushing it through from below generally doesn't work, so I just take a length of the old shock cord, double it and push it through from the inside back down the hole for that pole.
Put the length of new shock cord through the loop of old cord and then pull it up into the plastic hub with the old shock cord. Once you have a loop at the inside of the hub, you can remove the old shock cord.
Now you want to reclip the plastic latch onto the new shock cord. It might not latch, but don't worry if it doesn't so long as the new cord is all the way down at the base of the latch.
Now carefully pull the new cord out of the plastic hub, which is to reseat the plastic latch. This is a place where the 1/8" cord is a bit of a pain as you have to pull kind of hard to get the latch to return to its proper spot. Once it is in place, you can undo the vise grip and return the ridgeline poles into the plastic hub (this will prevent the plastic latch from going back out and causing problems as you fuss with the rest of the shock cord).
Now you want to put the short end of the new cord through the loose knot you made before. Tighten the knot by pulling on the cord on either side (i.e., not the loose end you just threaded in). Stuff the loose end, then the knot, down into the nearest (highest) of the pole segments. Once it is in the pole, you can pull hard from the other end of that pole segment to get the knot far enough in the pole that you won't be having it in the way when you are making your tent in a thunderstorm. (Be careful that your pole segment seats in the plastic hub properly before pulling hard at the end).
Now reassemble the full pole. Hopefully you have shock cord sticking out the end where the grommet-seating tip will be. Pull that cord out so that there is a bit more tension than you think the pole should have when done. Use the vise grip to hold the shock cord. Cut the shock cord 6-7" from the vise grip (you can always trim later, so err on the side of too long). Make a loose simple knot about an inch below the vise grip, then loop the free end of the shock cord into that knot, making a loop. Tighten the knot as much as possible. Now take the hook end of the grommet-fitting end of the pole and attach it to the end of the loop:
Now just release the vise grip and push in the knot into the pole segment (again easier if you are using 3/32" shock cord); I used the needlenose for that. Once it is in you can pull from the other end of that pole segment to get the tip properly seated. Once it is in the end of the pole, twist the pole to fully screw in the pole tip.
Wow, @cjones_cu! Thank you so much for sharing this detailed report of replacing your tent's shock cord. We will be sure to point other community members in the direction of this post if they come with questions about the Half Dome 2+!