Only comment is to keep from creating a "knee" joint in your columns. Make sure any cut and extension is restrained from buckling in all directions.Consider this, take and cut the posts one at a time at say four foot. Nail three 2xs the same width as the post say six foot long on three sides to the bottom 4 foot and screw the fourth side in. Put them 2 foot off the ground. Due this for all the posts. Now jack up the posts say a foot and insert a block to let it rest on by removing the screwed 2x. . Now fasten the top side of the boards to the raised posts and reinstall the fourth 2x.. The risk is minimal that the wind will blow it off while you are doing this. Borrow a few floor jacks and raise it a few inches at a time, block it as you go and work your way to the height you want. Second idea, I drove by a guy who slowly raised the roof on his concrete block garage using a different method. He raised the roof slowly till he had the height he wanted and then added a short 2x4 wall to fill the gap. There are multiple ways to do this. If you are nervous about the wind, wait for a still day and do what you can and then anchor the roof down with straps or chains via heavy weights or machinery until you can finish and tie it in properly.
I am no engineer but I am known to be creative, have a smart person confirm if these ideas have merit.