set up your old account from your 60gb on your new PS3 slim and redownload anything that you want again (it should all show up in your account on the PSN Store). Make sure that the old PS3 isn't connected to the internet while you're doing this though. When you've got everything that you want again, simply create a new master account on the PS3 slim and just stick to using that. Delete the old account that you set up on the slim if you need to.
In theory that should work anyway, I'll know for sure in about 2 weeks when my nephews PS3 slim arrives and I try to download Ratchet & Clank from my account for him