I went from a D300 to a D800e. I thought it was a very easy transition. There are a lot of new capabilities in the D800e, but you don't have to understand them to start using the D800. You can learn the new features after you start using the camera. Download a D800e manual now and read through it before you receive your camera. There is a pinned thread for an Excel spreadsheet in this forum that will help you configure your settings. Download the spreadsheet now and start planning your settings before you get your camera.
Many people seem concerned about problems caused by going from 12Mp to 36Mp. I think much of this concern is overblown. If you present your images at the same size as you did with your D300, you will get as good as or better images. If you start taking advantage of the extra resolution of the D800e or do a lot of pixel peeping, you most likely will find faults in your technique. Incremental improvement in your technique will take care of the problems. I'd plan on using the D800e on your trip, but I'd take the D300 as a backup.
The 24-70 and 70-200 lenses will work fine on your D300. The pixel density of the D300 is actually less than the pixel density of the D800. Just remember you have a 1.5 crop factor that makes the lenses seem longer on the D300. I think your Tamron and Tokina lenses are DX lenses so they will not cover the full image area of the D800e.
Yes, you will want larger and faster memory cards for the D800e. Each image file is 3 times the size of the same file on the D300. My old card reader worked on the new higher speed cards I got for my D800e, but I ended up buying a new USB 3.0 reader to get higher speed transfers into my computers.
To me an L-bracket is mandatory, but if you didn't have one for your D300 you probably can get by without one for the D800 too.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery