My husband and I want to fly LAX to Perth, departing mid-September and returning early October. I need to fly on United; we want to fly business; he'll fly on miles, and i"ll upgrade with miles & $$.

Right now, I can get this booking, with available upgrade for me on the LAX-Australia routing and business class saver (80K) flying into Sydney, and back from Melbourne for my husband. I can't get a guaranteed upgrade on the return if we go through Sydney; and I can't book him through on miles to Perth, at least not on United.

This means that in order to fly from Sydney to Perth, and then from Perth to Melbourne, I'd have to buy separate tickets (Qantas, or Virgin Australia). My question is: what would it take to change planes from an inbound United flight at Sydney to one of those other airlines; and then from one of those carriers to United at Melbourne. I don't know those airports. If we checked any luggage, would we have to claim it and recheck? Is that possible and, if so, how long a layover would we need? We could try to do all carry on, but we'll be there hiking, etc., for 3 weeks.

Thanks for any advice.