The page above discusses your situation, but the short story is that your battery is made up of older technology lithium ion batteries and the newer technology batteries have a slightly higher voltage rating. Yours is made up of two cells rated at 3.6V each, while the newer 7.4V batteries have two 3.7V cells.
If you do the math, the difference between 7.2V and 7.4V is about 3%. That's well within the 5 or 10% voltage tolerance rating for most electronic components. In addition, variations in voltage levels are always to be expected on any electronic device that uses a battery.
According to the Adafruit web page cited above, it's okay to use your old 3.6V (7.2V) charger with the newer 3.7V (7.4V) batteries; just don't try to use a 3.7V charger with the older 3.6V cells.
The short story here is that IMHO the voltage difference shouldn't be enough to cause problems with your Pill. If it was mine, I'd go for it, but you will definitely have to make your own decision on this.