Yes, it is necessary to remove the bolt that holds the damper rod into the lower fork leg in order to separate the leg from the tube and replace the seals.
You are on the correct path. Put the spring and cap back on, turn the fork over so the damper rod bolt is at the top.
The next step is key. You must use some sort of impact wrench to spin the bolt very quickly. A normal hand held wrench is too slow.
A manual impact wrench (the kind you strike with a hammer) usually will work. If not, a air-operated one will do the job. Just push down on the lower leg to compress the spring at the same time you hit the impact driver.
Once this bolt is removed the only thing stopping the tubes from separating is the fork seal. Remove the wire spring keeper above the seal and use the upper leg as a kind of slide-hammer to pop the seal out of the lower leg.
To replace the seal, you can make a tool from two pieces of PVC pipe that is split in half. Put one each half around the upper tube and tap the new seal into place.