Your numbers probably keep changing because of your network configuration. If you're using DHCP (which ismost likely if you're hooked up to a router to get on the internet) it decides on your address, but there's an easy way to work around that. Set the machine names to something easy to remember (and type) and you should be able to access the machine via the machine name with rendezvous. For example if the machine you want to connect to is called "thatmachine" you can put afp://thatmachine.local in the connect dialog. Avoiding spaces makes like a lot easier for this type of setup. You can assign the machine name in the top of the sharing control panel, (by default it will likley be _____'s Macintosh or something similar). Then connecting is done using machinename.local (the .local tells it to use rendezvous to find the machine which doesn't require knowing the numeric address).
Now the afp keychain stuff is nothing to worry about, you can safely agree to let it make the changes. It is just notifying you that your version of afp_mount was updated (it was part of the last security update) and it wants to update the saved passwords in your keychain accordingly to reflect the new version. Not being able to access it from pervious versions is not a problem, and I'm really honestly not sure why Apple insists on warning user of this. I've never seen a problem with doing this, ever as you almost never go back to using older versions of software like that