I did this exact modification. Not because i wouldn't have the green 3V cells, but because the driver wasn't regulating with them.
You need a DMM, then you need to turn the pot down (to lower current), then you need to connect it to the batteries through the DMM in current measuring mode, and then slowly turn the pot untill the current reaches 310-330mA.
I think clock wise is less current and counter clock wise is more current.
After that, the driver in your laser will have enough voltage on top to do the regulation, and the current will not change.
If on the other hand it is already regulating the current from two 3V batteries, then you don't have to change anything, and it would just work. But it's better to be safe than sorry.
If possible, you should measure the current with only 6V going in. Then you should turn it down, verify it is lower, and put in the higher voltage batteries. Then slowly and carefully raise the current again.
It worked for me, and now it has the same current untill the batteries are empty. Before it was not even in regulation, and the current and power were dropping with the battery voltage from the start. Some would appear to be capable of regulating from two 3V cells, but mine wasn't. But if it is regulating, higher voltage won't hurt it, as the driver will take care of it..
You can verify if it is regulating by using two 3V batteries and measuring the current as they slowly discharge. If the current drops, it is not regulating, and you should do what i described above. If it doesn't drop till the batteries are almost empty, it is regulating, and you can just put in the higher voltage batteries.
