I asked for jets first and helos second. I didn't even put multi down. I ended up with helos.
Truth be told, I didn't really want jets but if they were going to give it to me, I would have taken it.
For the most part, almost everyone gets their first choice. Despite popular belief, not everyone wants jets so if you ask for multi or helo, you're probably going to get it, especially helos. Top candidate will get his/her choice, the rest is a bit of a mystery. They told us that it had to do with ranking and a few other things, but on my course, the top 5 wanted jets and 1st, 2nd and 5th got them. 3rd and 4th went helo, their second choice.
I wish I could clarify the process for you, but it's a mystery to me, all I know is most of us got what we wanted.
As for switching after your first tour, it really depends on your community and timing. For instance, in the next 4 years, we'll be getting the Cyclones to replace the Sea Kings, I most likely will not be posted out of the MH community, or so I'm led to believe. Suffice to say, it has been done in recent memory, I know a few ex-Sea King guys that are now flying Griffons.