if you have had the bike since new, never added fluid, you can leave the top on..
I would replace the pads without pushing the pistons back yet..
And when you install the caliper, push them (the pads) back only far enough to fit over the caliper.. if while pushing the pads back it goes "rock hard" crack a bleeder open and run some fluid out.
keep in mind any fluid you spill on paint is going to be bad news.. make sure you take precautions to not allow any fluid on any plastic or paint..
if you are not sure about fluid being added, I would crack a bleeder and drain about an ounce out and then install the pads (still leaving the cap on)
Now that you have the pads installed, take this as a good time to flush the fluid out of the system..
Run a tube off the bleeder into a cup, open the cap and run about 8 ounces of fluid out of one side and around 4 ounces out of the second side (does not really matter which side goes first)
Just a bit more to brake service than slapping fresh pads on if the bike is more than a year old..