In my experience of finding my type and types of my friends the J/P switch for introverts is true. Some will tell you otherwise but this is how it work out for me.
J/P switch is neither reliable, valid, nor advised since:
1) There is no direct structural correlation between the systems, thus a functional nor letter-by-letter method of converting can be definitive.
2) The semantics and perspective of the functions, type and system in general are different, especially in subtle ways that aren't grasped until having significant experience.
3) There is no way to be completely sure about your MBTI type, thus carrying it over with confidence to another system would increase the unreliability.
Even though it's most likely a person will fall within a close range, Socionics type should be assessed from scratch to prevent misunderstandings of Socionic theory.
Introduction into Socionics (draft): Part 3
Comparative Experiments and Their Results,
or Measuring the Difference between the Socionic and Keirsey Types
We proposed the 16 descriptions of the Keirsey types to 108 socionists (this means, each of the 108 read ALL THE 16 descriptions), and we asked them to identify the socionic types in these descriptions.
The table below represents the result of this experiment:
[BIMG]http://i.imgur.com/AbyM0.gif[/BIMG]
And the next table represents one more result of this experiment. We asked the participants to indicate their own types, and to recognize their own types in these descriptions:
[BIMG]http://i.imgur.com/TC1FR.gif[/BIMG]
Do these tables represent the real correlation between the socionic types and the Keirsey types? We think they do not. They rather represent characteristic stereotypes of the socionics and the Keirsey typology. To compare these typologies objectively, we will need to test at least several hundreds of persons using both socionic and American methods. But at least we know now for sure that socionics, MBTT and Keirsey, in spite of their common origin from the Jungian typology, are not identical!