There's no rule that says that can't all be changed to double sockets. (After all, you can plug a 4-socket, or even 6-socket, extension lead into a single socket and then, if you wanted to, plug further multi-socket extension leads into them. It's not the number of sockets that matters but what you plug into them).
Each ring circuit in a domestic property normally has either a cartridge fuse (rated at 30A) or a circuit breaker (rated at 32A) protecting it. You can plug dozens of low-current devices (such as phone chargers) into the sockets on any ring circuit without exceeding the maximum load. However if you were to plug just three 3kW heaters into sockets on the same ring circuit, the fuse should blow or the circuit breaker should trip.
So, as long as you're aware of the maximum total load which can be plugged into any one circuit, there's no reason why you can't switch to double sockets.