I have had a number of reloaders where the head moved without any resistance over its normal range of motion, but seemed very tight when removing and reinstalling the tool head. Most of the time this is simply because the keyway slot is more worn where the key in the tool head usually travels, and less worn at the top of the shaft. If you really think that the key is buggered up, I would use a jewelers file to smooth it out rather than try to remove the key. Getting that key, or a new key back in might prove to be more difficult than you expected.
On some machines, the hole in the shaft through which the taper pin is inserted can get deformed, resulting in resistance when you try to move the tool head over that spot. You can clean that up with a fine swiss file.