Schalke and Bremen just got relegated last year from the Bundesliga. I think both have enough people in their team with Bunsesliga experience who will enable them surviving the Bundesliga season next year. Of course they will need some new players, but they will also have much more possibilities financially in the Bundesliga.
I dont know if the experience from the players of Schalke and Bremen will get them survived in the first german league.
Just look at Stuttgart and i guess i have nothing to say about Greuther Fürth , its diffrent when you are in the first league.
Also the financially will be a bit more expensive as in the second league , there is so much more going on in the background with costs and stuff.