What is the best way to make an updater?

[rumovoice]

I've made a setup that has only one default section and a bunch of MUI2 pages. Now I want to make an update mode (command line switch '/update', updater is downloaded and launched by a program itself) that will hide all MUI pages and display only progress bar. Also updater has a completely different code so I want to make a different section for it. What is the best way to do it?