Depending on your roof structure, you may be able to cut out the old ridge pole. And then slide in a new one. Most likely the ridge pole does not hold up the rafters, if I get the idea of the roof structure right. On our log cabin, the rafters are supported by leaning against each other. The ridge pole was convenient originally, and held the building together along the length.
But now, it would be possible to take our Poplar (balsam?) approximately 10" to 14" (varies) diameter ridge pole out (I would have to use a "sawsall" to cut the 8" spikes that go thru the 6" diameter rafters into the existing ridge pole first) and the roof would still be supported - in our log cabin. Then I could slide another one in (with a big crane) - most likely at the same time - remove some - replace some, etc.
If I wanted to do it myself, I would roll a new ridge pole up on poles leaned agains the existing roof edge, and then roll it up the existing roof - that's how I got it up there at first - only I hadn't put the roof on yet. I used a big 1" diameter rope, fastened to the purlin on the far side, then wrapped it around the log on that was on the ground, and took the rope up and over to the other side and pulled with our little tractor, you could use a truck. It rolled right up real nice, just like all the other logs, with a little adjusting on each end of the log with a "cant-hook" to roll the log on one end so it was level.
Next I would brace the existing rafters temporarily so I could cut out one side of the rafter, then cut out the roof at the ridge, cut out or roll out the old ridge pole, then roll in the new ridge pole, and then fix the rafters and roof, and then finally remove the temporary braces across the rafters.
I do not think that I would splice a ridge pole - I would get a new one. Our cabin is 20'-4" x 28'-0" inside, two story, with a 45 degree pitched roof. We have about 4' over hang each end. Our ridge pole is about 36'-0" long and as I said about 10" to 14" diameter.
We spiked our ridge pole into the ends of the cabin with multiple 12" long spikes. I also left our ridge pole and purlins sticking out beyond the roof 30 years ago - didn't know any better - seems so clear now that they can't stick out. Now I cut the off and coated the ends of all logs with a deck sealant material - its gray, I painted it on with a roller on a long pole.
Ridge Pole, Purlins, and Rafters Leaning Against Each Other
Ridge Pole And Purlins Cut Off
Good Luck, Pretty big job.
Alex