Only started today but it seems like a good sim.
If I upgrade before the end of my 7 day trial can I keep my airline/progress or must I start again from scratch?
Yes, you can seamlessly keep playing the same airline after the trial period all the way until the end of that particular game world. (though that BW#1 game will in about 10 days; they last for about 1.5 months alltogether).
You might consider switching into BW#2 game world instead as that has just started. (it has the new cargo features too)
Can I switch with my airline or is it start again in BW2?
You'll have to start again. Because timelines are different, settings also, and as Sami said the new cargo feature will be introduced to a BW for the 1st time (that feature is just a few months old, with completely new dynamics - city based demand - and for that part I'd advise you to read the manual well).
Ok thanks.
I'll stick with what I'm doing and go from there