Another group seems to believe that, the further into endgame you get before the expansion hits, the less green quest gear you'll need. Naxxramas level gear was good enough to roll straight into Karazhan with, having comparable stats and DPS to the level 70 instance blues others ground for. So if you're killing Illidan, Archimonde and Kil'Jaeden regularly than you'll probably be able to vendor or disenchant a lot of the quest items and worry less about the gear reset, more about getting ready for the new endgame. This is, of course, an argument for trying to switch mains before the expansion so that you'll be geared enough to avoid any gear reset.
I'm not in either of these groups, but I think they both have valid points.
I'm not switching mains before the expansion because my human warrior is one of our best geared tanks, and I'm not costing us progression raiding over my dissatisfaction with my racials (okay, okay, that part is a joke, human racials are dynamite) and my race's appearance. Yes, three years down the road I'm pretty sick of looking at his ponytail and mustache, but the other 24 or so people who show up on raid nights deserve the best geared tank I can provide for them.
However, I am taking steps to make the gear reset as painless as possible for me. I'm PvPing on my draenei (both battlegrounds and arenas) to try and collect a solid set of gear for heading into Northrend. I always bring my draenei instead of my human to Karazhan because by now Kara is primarily an alt/badges run and so I don't really need to be all that well geared to run it, we have plenty of alt tanks who'd like to pick up some badges. It's also fun for me to play around with arms/fury and fury/prot specs on him, using Kara and heroics as my testing grounds for when it's not feasible for me to switch from heavy protection specs for our progression content.
I know I'll still reset gear in Northrend. I still remember when my Sulfuras was replaced by a green sword from Nagrand around level 66. Honestly, I probably could have held onto Sulf for another couple of levels. The point is, while I believe that the argument that better gear will last longer into Northrend is factually correct (you didn't replace Tier 3 with quest greens, or even instance blues, you replaced it in Kara) for someone about to switch mains the idea that you don't really have to worry about switching is probably valid enough. I just want to try and stack the deck as much in my favor as I can and have fun doing it. It's really a blast to play fury in instances after tanking in raids, it's a total stress release.
I'll probably end up leveling my human to 80 so he can be my fury alt when my draenei is tanking the new endgame, in fact. But we all knew I was nuts anyway. Are you thinking about switching mains? How are you planning on going about it, just logging on the day of the expansion and hopping a boat or are you running stuff in preparation?