It can be done. You have to remove the forks from the trees either way. Before you remove the forks loosen the fork cap. Need to loosen the upper tree bolts first or the cap won't come loose. Remove the cap. With forks fully extended fill with oil up to near the top of tube. Just room enough to replace cap.
Remove the fork from the trees. Slide the rubber boot on the top of the lower part of fork. Under that boot you will find a snap ring. Remove this ring and the spacer under it. You should be able to see the seal.
Now prop the fork some where like next to the deck of your house. The bottom of fork on something solid. With the top about even with the bottom of the deck. With a long 2x4 put one end of 2x4 under edge of deck. The 2x4 should run out across the top of fork. No lever the top of fork down with the 2x4, With enough pressure on the 2x4 the fork compresses enough to blow the seal up out of the fork.
Remove the fork cap, pour out oil. Replace the cap.
To replace the seal, slide the seal down the tube to top of the lower. Place the spacer on top of seal. Get a piece of PVC pipe big enough to slide down over fork tube. About the same outside diameter as the seal and spacer. The PVC pipe should be long enough so it stick up above the fork tube by a few inches. Now push down on PVC to press the seal into place. Might have to gently tap PVC with a mallet. If no mallet a piece of 2x4 will work. Just be gentle. Don't want to hurt the seal.
Once the seal is in place put the snap ring back to hold the seal. Put the boot back on.
At this point I would flush the forks with a suitable cleaner. Kerosene or fuel oil, Diesel even. Put in a cup or so. With the spring out. Collapse the fork and extend it to work the cleaner through the damper and other internal parts. Drain, Repeat till the cleaner comes out as clean as it went in.
Using oil flush the forks as described above. This removes the cleaner. Then refill with the amount of oil you choose. I like the 6 inches down method of filling the forks. With the fork collapsed, put in enough oil so the tp of the oil is 6 inches down from top of fork tube.
Easy way to do this is pour in about 8 to 10 ounces of oil. Extend and collapse fork a few times to work out the air. Take a piece of clear plastic hose, put a zip tie around the hose. Set it to 6 inches from one end. Slide this down into the fork till the zip ties is at the top of fork. I have a Miti Vac, I hook the Miti Vac to the hose and use it to draw out any oil above the end of the hose. This very precisely sets the oil level.
I have read about this method of seal replacement. It is very messy, Gets oil all over the place when the seal comes out.
You have the forks off the bike and half tore down any way, so why not tear them completely down and check the ingternal parts and clean them properly.
Leo