Syncing apps on another computer

[Bellboy]

My son recently updated from his iphone3G to iphone4 and gave the old one to me. I went to the AT&T store and got it turned on to work on my plan. The phone works ok but I am having trouble with syncing the apps. I have my music, photos and contacts moved into the iphone but it balks when I click to sync the apps. I have downloaded two apps to itunes. When I click to sync them a message comes up that says:
"Are you sure? All existing apps and data will be replaced with apps from the itunes library."
Since I only have the two apps loaded in itunes, will it erase all the apps on the phone? Even the ones that come on it? Or just what has been added since it was new.
Also the apps pages for the iphone are grayed out so you can't rearrage or do anything with them.
What can be done to fix this?

 

[Gregoris]

did you authorise the iphone with that computer?

 

[Bellboy]

Well I have set up an account with itunes. I think the reason it won't let me arrange the apps (the app screens are grayed out) is because some of the apps were purchased on his account.
If I go ahead and try to sync them anyway, will I just lose the ones he has added or will it take all the apps away?
Everything else is working ok... music, pics, contacts, etc.
Does itunes still want me to authorize something else? How can I make it forget the other computer and account?
Oh, I have os 4.2.1

 

[Bellboy]

Ok, I have it working like it is supposed to now. First I backed it up so if things went wrong I could get it back like it was. Then I went ahead and synced it and let it remove all the apps that my son had installed on it... there were 8 of them. After that I put a check mark on the apps I had downloaded and let it install them. Then finally I moved all the apps around to how I wanted the screen to look and did a final sync. Everything is right like I want it now and all my apps work.
The help instructions in itunes and for the iphone leave a lot to your imagination as to what it means.