Changing car locks to newer ones for security?

Installing an aftermarket security system (possibly even two-way or with cellular communications and a GPS-tracker) would be your best bet if you are concerned about car being stolen. Locks are often model-specific and I don't believe it would be an easy task to retrofit a newer lock into an older car.

Generally, any car lock, new or old, can be opened very quickly if the person doing it is skilled enough and is significantly motivated.

@Enot has an excellent answer but I still want to contribute a little as well.

Specific to your questions, Auto locks made in the 90's I would argue are BETTER in many cases than their newer after market counterparts these days. Many older locks used side bar designs (like the GM door locks, Ford ignitions, Chrysler locks) where the newer replacements are made cheaply as basic wafer locks and do not use side bars (and even use LESS wafers in some cases).

In general, Even if you put a unpickable lock in its place then it would still do you no good. 98% of automotive lockout openings completely bypass the lock cylinder for entry all together.

Door locks have become very basic in their level of security and only prevent someone from opening just as they were passing by randomly or allow the cars owner to open the door when the battery is dead. Really that is all they need to be able to do these days. Entry access has gone to keyless remotes and security has gone to advanced alarms/immobilizer systems shutting the cars down.

It makes no sense to change the locks in your car, because it isn't locks that prevent a car from being stolen. Door locks can be bypassed with a slim-jim, or a good old-fashioned brick will smash a window. If you want to prevent your car from being broken into don't have anything valuable within sight, if you want to make your car hard to steal then invest in a steering wheel lock or engine immobilizer. Neither of these is foolproof, but they make it much harder to steal a car.

With older cars, the value of the securing the vehicle is not in preventing it from being stolen, but rather preventing the contents of the vehicle from being stolen. Other than joyriding, most cars from the 1990s would have little value to a thief. There are almost no demands for parts for vehicles from that era, unless someone just happens to have a specific need for a specific part from a specific vehicle.

That is why in some places, such as the desert in Australia and the Negev in Israel, many people don't even lock the car. The real threat is to the vehicle contents, and for that thieves have no problem smashing windows. I've even seen the windows left partially down to prevent them from being smashed by thieves.

That said, catalytic converters are commonly stolen from all vehicles, especially trucks as the cat is not only larger but it is easier to access. Exhaust shops can weld in a cage to protect the cat if that is a concern for you.

Ironically for the OP's question, the 90s is where cars started to be harder to steal because of the widespread addition of immobilisers. These were required by law in Europe from 1998, but most cars had them before that. I know my 1993 Peugeot 309 had one.

As other answers have said, with the addition of immobilisers, theft of cars became relatively rare. Immobilisers can certainly be cracked by more professional thieves, but there isn't that much profit in stealing a regular car. Most of the issues are theft from the car, and a simple brick will get you into the car to steal stuff. Vans often have more substantial aftermarket locks on the back and do not have windows that can be broken, but on a regular car there's really no point.