What will happen when I install Leopard?

Question

So when leopard comes out in a few weeks and I install it, what will happen to all the files and apps on my computer. Will they be gone? Will I have to redo everything?
This is my first transition on a new OS X. I switched right aftr Tiger came out. What is going to happen?

Answer 1

Fair comment!
If you want the full nine yards:
There are no guarantees, but following this procedure when installing updates and upgrades on your Mac will go a long way towards avoiding unpleasant after effects and ‘post-update stress disorder’.
It is also worth noting that it is an extreme rarity for updates to cause upsets to your system, but they may well reveal pre-existing ones, particularly those of which you may have been unaware. If you are actually aware of any glitches, make sure they are fixed before proceeding further.
So before you do anything else:
If you can, make a full backup first.
Turn off sleep mode for both screen and hard disk.
1. Repair Permissions (in Disk Utility)
2. Verify the state of your hard disk using Disk Utility. If any faults are reported, restart from your install disk (holding down the C key), go to Disk Utility, and repair your startup disk. Restart again to get back to your startup disk.
At least you can now be reasonably certain that your system does not contain any obvious faults that might cause an update/upgrade to fail.
3. Download the correct version of the COMBO update from the Apple download site. If your car runs on gasoline you would not want to fill the tank with diesel, so don’t try to install the PPC updater on an Intel Mac!
If you prefer to download updates via Software Update in the Apple menu (which would ensure that the correct version for your Mac was being downloaded), it is not recommended to allow SU to install major (or even minor) updates automatically. Set Software Update to just download the updater without immediately installing it. There is always the possibility that the combined download and install (which can be a lengthy process) might be interrupted by a power outage or your cat walking across the keyboard, and an interrupted install will almost certainly cause havoc. Once it is downloaded, you can install at a time that suits you. You should make a backup copy of the updater on a CD in case you ever need a reinstall.
Using the Combo updater ensures that all system files changed since the original 10.4.0 are included, and any that may have been missed out or subsequently damaged will be repaired. The Delta updater, although a temptingly smaller download, only takes you from the previous version to the new one, i.e. for example from 10.4.9 to 10.4.10. Software Update will generally download the Delta updater only. The preferable Combo updater needs to be downloaded from Apple's download site.
Now proceed as follows:
4. Close all applications.
5. Unplug all peripherals except your keyboard and mouse.
6. Install the update/upgrade. Do not under any circumstances interrupt this procedure. Do not do anything else on your computer while it is installing. Be patient.
7. When it ask for a restart to complete the installation, click restart. This can take longer than normal, there are probably thousands of files to overwrite and place in the correct location. Do nothing while this is going on.
8. Once your Mac is awake, repair permissions again, and you should be good to go!
If your Mac seems slightly sluggish or ‘different’, perform a second restart. It can’t hurt and is sometimes efficacious!
9. Open a few of your most used applications and check that all is OK. In this connection please remember that not all manufacturers of third party applications and plug-ins, add-ons, haxies etc, will have had time to do any necessary rewrites to their software to make them 10.4.10. compliant. Give them a weeks or two while you regularly check their websites for updates. This applies particularly to plug-ins for Safari 3.
N.B. Do not attempt to install two different updates at the same time as each may have different routines and requirements. Follow the above recommendations for each update in turn.