Can you transfer all data from an old account to a new account

You'll have the option if you want to overwrite or not.

When copying data using the Save Data Utility, if the destination (whether the PS3, a storage device or the PS+ online storage) already has a save of the same game (on the same slot, if applicable), you'd see a prompt asking you if you wish to overwrite the save, so you can do the copy process without worrying of accidentally overwriting your save.

However, using a system transfer or restoring a backup completely removes any previous data on the target console, so if you wish to keep the saves on your new console, make sure to copy them beforehand to a storage device or PS+ storage, then restore them after the transfer/restore completes.

All you have to do is activate your new ps3 with your psn account. You can have up to 2 active ps3's per account, for cases like this is the thought behind it. Once you activate the ps3 with the account you can redownload your dlc's through the playstation store with no problem at all. So as long as you didn't give a friend's ps3 access to your account you will be AOK. If you did then you can force deactivate all ps3's or a selected one on your account through the SONY site or ask your friend to deactivate your account on their ps3. I did this once they restricted the number of active ps3's to 2.