I have an upcoming trip that is currently two one ways of JFK - HNL - LIH (then the reverse). I booked main cabin with cash and upgraded to first with points on both flights. Is it possible to change this to just be the JFK to HNL (and then HNL to JFK) flight?
I called support and they recommended skiplagging the first segment (not even sure that is allowed) and that I would need to fully cancel the return flight and rebook economy, and then reupgrade to first with points. But now no points upgrades are available of course.

I have had HA support give me so much incorrect information in the past so I am not sure what to trust, but does anyone have any experience with this? Is it possible to sort of just drop the interisland flight without fully cancelling and rebooking (and in turn losing my upgrade)

Sounds like you already tried to drop the segments and they told you no. I suspect that's the answer. HUCA isn't usually a rewarding strategy with HA because their agents have little to no discretion to vary policies. One option for saving the upgrades would be just to change your HNL-LIH segment on the outbound to shortly before your return flight and return HNL-LIH-HNL-JFK, That would waste part of a day but save the longhaul upgrades.

My main concern is the support is just wrong. The last 4 times I've called (to upgrade) they were incorrect on point upgrade costs and availability. One agent tried billing 80k, another 70k, so I am just unsure if this is actually impossible or the agent was simply incorrect. I was also told upgrading these flights in the first place was impossible a couple weeks ago

If I need to keep the LIH sement its no big deal, I just wanted to see if it was even an option to switch

You could skiplag on JFK-HNL-LIH without impacting your return since they're separate tickets. If you do decide to skiplag, make sure you are all carryon because you likely won't be able to short-check any checked baggage to just HNL. I can't speak for HA specifically but generally airlines will not allow you to short-check checked baggage unless the layover is of a certain length or an overnight layover.

Your return is more difficult because you generally can't show up at the midpoint and skip-lag the first segment (LIH-HNL) and still fly the second (HNL-JFK). You either have to change to just HNL-JFK and give up the upgrade (there's always the possibility that when your upgrade is pulled the inventory is returned and opened up so you could upgrade again but that's not a guarantee) or you could always just take an early flight to LIH ahead of your LIH-HNL-JFK return which would be an inconvenience and added expense but would preserve the upgrade on the longhaul.

For anyone searching later on, Hawaiian Airlines did let me drop the Kauai segment and remain in first class without cancelling and rebooking everything.Just need to find the correct agent