I would try apply a little penetrating oil along the edges... wait... repeat... wait... etc... penetrating oils are super-low-viscosity oils (I use "Liquid Wrench", but there are lots of brands) and are great at seeping into the threads through the tiniest little crack/space... getting the lubrication into the threads where you need it. just give it some time (overnight?) to do its job. The good news for you is that unlike some threaded fittings that bind up, these fittings aren't very deep, so the oil doesn't have very far to travel/wick to do its job... sometimes you've got corrosion on a bolt that's a couple inches away from where you can put the oil, and that's much harder. Peggies advice (as usual) sounds similar and good too... I'd start with lubrication before you start using brute/mechanical force... less likely to strip/damage things that way.