I think many people are answering as if it's some gift or super power only you have. If this is a result of a commercial technology, I'd expect everyone around me to have the same options, and am going to answer with that assumption in mind.
1.If you could expand your lifespan permanently via scientific-technological means, would you do it?
Yes, assuming I can opt out of the service and kill myself if I one day choose too. As long as life isn't predictably hopeless, then you have one of two states - either life with a reason to live, or life with the potential to find a reason to live tomorrow. Otherwise, even if nobody is making the choice to live forever, almost everybody is making the choice to live a bit longer and not suffer from whatever it is that might be killing you otherwise, and to not do the same would be to abandon everyone around you. But, if this is an outer-body service (like resurrection in computer games), then I want the option to opt out, because I can't reliably predict that I won't find a counter argument for this by the time I am 5 million years old.
2.If you could modify your central nervous system to eliminate all pain, would you do it?
No, but I might like to change the symptoms and experience of pain so that it isn't as debilitating. Maybe give it the settings of an alarm clock, so that you can go "I hear you, I am trying to fix the problem that's causing you, now shut up". Like a pain snooze button. Maybe make pain more informative so that you have a better grasp of what is causing it - less like a noisy siren in your nervous system and more like a developed addition to the 5 senses. Instead of getting rid of pain, make pain work better.
3.If you could modify your central nervous system to put you in a constant state of euphoria, would you do it?
No, but I might use it as an incentive system to give myself an euphoric response to things that I need to do but don't really like doing.
4.If you could drastically change your body to something different, would you do it?
I'd like to have a wardrobe of bodies to which I can move my conscious too. Going out for a run? Time to be a centaur. I have time off? I'll take on something that can fly. Going out partying? let's see... Am I in the mood to look like brad pit or like a Japanese tentacle monster? And what race do I want to be today? African? Asian? How about native american?
5.If you could drastically increase your intelligence, would you do it?
If everyone around me has the same ability, and I want to be employable and relevant to the economy, do I even have a choice?