Ps3 – How to keep using DLCs purchased on an old account with the new account

I have done that myself.

Since i'm not from USA, i created a different account i buy credit for it to buy some stuff. The only restriction is that in order to play or use the stuff you buy with that account, is that both are as users in the same PS3 system.

This is necessary since it has to check for the user who bought it, but you can freely use the games/stuff for your main account

At present, there is no way to transfer trophy information from one account to another (still). Logically, it doesn't make sense for trophies to be transferable. This person earned this proof that they did something - what meaning would these have if you could transfer them to someone else? This would certainly allow for a shim to act as a psn id change, but that would be the wrong solution to the problem.

Some few saves and other data can be copied over but often, like trophies, these are usually non-transferable as well. To determine which data can be transferred, you would have to go through each and try it. The logic behind this is probably the same as trophies and possibly even related to how much easier it is to cheat trophies with someone else's saves.

You still cannot change your psn id and there are no practical workarounds. All you can do is start over. Sony might have reasons, but it is a hotly discussed feature that exists on the Japanese PSN. There has never been an outright denial of the future possibility (only statements that implementing this feature is "very technically complex").

On PS4 you can sort of get around this by changing your real name, but that doesn't allow numbers or special characters and will not help you on PS3.