Ahhhh the enjoyable vagaries of AA awards
. My expertise is being a third year AA EXP and having redeemed some absurd high six figure number of AA miles over the past few years
.
First off, OP, is there a single fare basis SGN-BOI on any carrier that allows either JL or CX as the overwater carrier as well as routing to BOI? AA hasn't served BOI for a while, so you might have some difficulty with this. I haven't checked Expertflyer or elsewhere to see if such a fare basis exists. I would consider calling AA and attempting to book an economy award if you aren't sure, put it on hold and simply let it expire and you should be able to find out easily enough.
Next off, as
@Rich notes, airline changes are free, along with routing and date changes. If you book a Saver award A-B, you can change from any routing to any other legal routing (i.e. meeting the above conditions of published fare and allowed routings and airline combinations), or date, or airline, for free; as long as A and B do not change and all travel is completed within 365 days of initial booking date. If A or B change, that is where the fees come in.
I personally like JL a lot, but the JL 788 J is angled flat according to what I've seen, while CX 777 J is lie-flat. I've also only flown F for long haul on either airline. If you sleep well enough in angled flat (I sleep poorly in almost all cabins
), I would go with JL J. If not, go with CX. Of course, CX also means less time in regional product, unless JL operates a long haul product to SGN (haven't checked and don't know off the top of my head).