Liquids to gases

To make a philosophical point, mind uploading would in my opinion create a new consciousness rather than transfer a pre-existent one. A copy of your consciousness would remember experiences which could be transferred back but that's not the same as going through it. This brings up my concern with the practicalities of virtual reality, as it happens, because whereas a suit, goggles, headphones and so on can mimic some of the sensations of inflation, they can't provide the internal sensations, so some kind of hack to the nervous system would seem to be required. It just wouldn't be the same otherwise, although a VR suit which provides visual, auditory and external tactile sensations for inflation would be pretty brilliant tbh.

Concerning body modification, yes i can see that working, possibly in the form of multiple inflatable implants or one large one.

Finally, concerning pain i try to have it both ways here. I think it would start off painful but the pain would fade due to the pressure cutting off the circulation completely, so it would be like going numb all over, which as it happens does feel like turning into a balloon to me. I can remember someone describing having a dental anaesthetic to me when i was a child as feeling like her face was "turning into a balloon", which at the time i found quite exciting. Also, when i've inflated myself it's painful afterwards, not during, because of the colic. The way i'm having it both ways of course is that i'm simultaneously trying to claim it's physically sexually stimulating and that it reduces sensation, and i can't have it both ways!

There are liquids with these properties but they're not inert. For instance, some essential oil components are like this but have a major pharmacological effect, and ether is the best example of all and would either cause unconsciousness or kill. So, "continue the research" i think.