You think about the choices available to you and make up your mind.
"Look, mister, ... or miz or ... whatever it is you are ... I am so done with this human body. I got the short end of the stick, as you can see, and if you can turn me into something else go for it. Being human sucks even if you can walk. I don't care what you turn me into, just as long as it can walk."
The alien seemed delighted that you made this choice.
"That's wonderful Alexander," he answered. "I am so glad you have volunteered for our corporal trans-species reconfiguration experiment. As I promised, you will be continue to be sentient. I'm not sure whether you will be able to 'walk' in the way humans think of walking, but I promise you will have a physically capable body by your new species standards. Your strength mobility be above average. After we get plenty of data, you might eventually even be transformed again. First we have to insert a nanochip into you coordinate the change."
A little pinprick happens in your right buttcheek. "Ow!"
"Now our scientists are programming the bio fluid ... Done!"
A burst of green goo washed out of the walls and drenched your naked body before being absorbed back into the floor. You stood their shivering while the voice continued.
"Now just sit back and wait for your change."