7.4 volt mah 1200 battery can you use 7.4 mah 2000 instead?

Depends.

Batteries have two main features, their voltage and their capacity.

Both your batteries have the same voltage, 7.4 V, and that's a good start.

But they have different capacities.

If they were fuel tanks, they'd hold different amounts of fuel when filled.

The 2000 mAh (Milli Amp hours) is obviously bigger.

If the batteries are reasonably similar in their internal construction, whatever you're planning to run from the battery will probably do just fine with a bigger battery.

Just like a car or outboard engine won't mind being hooked up to a bigger tank.

There are two things that MIGHT happen though.

Batteries can also have different output currents.

Some, like a car battery, are good at delivering huge amounts of current for a short time.

They have to be, because it's what's needed to turn the starter motor.

Other batteries are better at delivering a small current over a longer time.

Back-up batteries for fire alarm systems and things like that do better with that kind of battery.

So if the 1200 battery is meant for a high-and-short discharge, replacing it with a low-and-long discharge unit probably won't work very well. That can cause damage to the battery.

The other way around - using a high-and-short discharge battery for a low-and-long discharge set-up doesn't damage anything, but the battery won't last as long between charges.

The other thing that can happen is that your charger might get upset.

Generally, the charge current a battery will (try to) pull from the charger is fairly dependent on the capacity of the battery. Bigger battery = bigger charge current.

If your charger is closely built only to deal with a 1200 mAh battery, it might not be entirely happy trying to charge a 2000 mAh battery. That's 2/3 bigger.