The subjunctive is the mood for describing the things which do not exist as such--but don't not exist either. It is great for describing possibilities and hypothetical situations.
In your example, I can't say 'if I was you' because I am not you, nor could I ever be. To use the indicative mood (if I was...') would suggest that such body swapping is a definite reality.
The sunjunctive is the grammatical equivalent if a Magic Eightball saying, 'outcome uncertain, ask again later'.
I don't know if that explains it but maaaaan it sounded deep.