I have a flight from BOS->SEA->HKG coming up. I had booked the flight in economy Q fare. I called Delta and was placed on a priority upgrade waitlist for 70k miles (if it goes through of course).
But what if only the BOS -> SEA leg got upgraded and the SEA->HKG didn't. Would I lose that 70k miles?

I'm platinum and would be a shame to lose 70k miles on something that I could potentially have just been upgraded for free on. There is no way to choose the waitlist to be applied only on SEA->HKG.

Anyone experienced this before?

Okay - just called Delta to clarify:
If only BOS -> SEA is upgraded then it's 25k miles
If only SEA -> HKG is upgraded, it's still 70k miles (same as BOS - SEA - HKG)

So worst case scenario, I would lose out on 25k miles for something I MAY have been upgraded for free.

What would you guys do?

You should call back and only waitlist the long haul portion. That is probably the only part you really care about.

I would be extremely surprised if you got upgraded to F on BOS-SEA. I have found it extremely unusual for F cabins to/from SEA on TCONs to not be filled with paid pax. There might be 1 or 2 DM that will get an upgrade at the gate, but even that is unusual these days. I doubt it would be a waste of 25K miles.

Any TCON to SEA F upgrade is hard to get as all TPAC D1 pax will be connecting on inbound F.
SEA HKG is quite full in D1 except for summer holidays where business slows down as everyone is home or away on vacation.

Thanks, guys. Will let you know how this plays out on Sat.