OP is not feeling guilty but utilising positive personality traits regarding helping old friends and sharing of resources. I reckon if he explained the situation, the CSM would be quite amiable or even impressed by his altruism and upgrade his friend.
Well, an upgrade would be nice but upgrades are in the lap of the gods. Like the weather; it happens or not and there's not much control over the situation. Even when submitting upgrade requests it's a bit of a gamble.
I've swapped my J seat with a Y passenger a couple of times in the past. Once on the old QF63 when it was a 744. I was travelling with my wife somewhere. For me it was the tail end of a DONE4 and we shared legs. I got the CBR-SYD up front and she had the SYD-JNB leg. No problems, just tell the staff what we're doing.
Letting my friend have the first half of the flight minimises disruption, I think. Playing with the controls is one thing, but trying to recline a no-recline seat could be tricky. Best to sort that out with the lights on and FA assistance on hand. I've flown on these seats before - and in fact will have just flown up from SYD in one - so I know how everything works.
My thinking is that J pax are able to go back and visit the Y cabin and she can have a solid sleep and do just that, waking me - hah! - halfway through the flight, and then I am just a J pax returning to his assigned seat, BP in hand. So long as the cabin crew know what's going on there should be no problems.
On that note. I have a strategy. My SYD-SIN leg is in J on one of the Finnair wet-lease aircraft. I think that once there's a quiet time after the meal service, if I poke my head into the galley, a pack of Ferrero Rocher in hand, then I can ask about the Finnair policy on mid-flight swaps. I should be able to get some current and useful information to inform my next step. These will be Finnair cabin crew, routinely assigned to premium cabins, and most likely they rotate through the system, maybe working the SIN-HEL leg the next day after a rest. They will know what goes on, what's allowed, what my chances are.
In SIN, I can approach the staff at the gate lounge desk prior to boarding and inquire, based on what I've already learnt, possibly with a pack or two of Tim Tams at my disposal.
I really can't see how there could be any problem in merely swapping seat assignments for the whole leg. I'm actually prepared to let her have the whole flight up front. After all, I get the points and the credits and we'd be in the same position on arrival as if I'd done nothing, just with the roles reversed.
The tricky part will be the bold mid-air transfer and I think some chockie bickies might smooth the way.
