Guys, my best advice for you is that you come up with a way to get up and down the mast that you are comfortable with and get used to being up there.
There is really no way to know what you are going to run into when you set about removing the old fixtures. In all likelihood, fasteners will shear off or the heads will strip out. You don't have any idea of the condition of the wires or if you have enough old wire coming out of the mast to work with. You have to figure out how to mount the new one where it will be visible and not interfere with the Windex etc.
Are you going to drill and tap, use rivets, or God forbid, self tappers?
In other words, it's probably going to take you a few trips.
Get used to being aloft before you attempt a real job.
I carry a crap load of tools and materials in my chair and it never fails. Almost every job I do, I still have to have someone send me something up.
Led's are a good way to go, but they're expensive. There are some that will operate as a anchor light as well as a tri-color with only two wires.