So what can you do? Prepare.
1) Money: there will still be vendors in Northrend and people will still be selling their stuff for money. According to WotLKwiki, most of the profession-crafted items will be BoE, so you will be able to buy these items (or their ingredients). You can make money from daily quests and AH trading. The more you have, the more items will you be able to buy. Maybe you can do some big investment that makes you rich when expansion (or the patch before) comes out.
2) Honor: Blizzard canceled the plan of deleting honor points when expansion comes out, you will be able to buy PvP items on lvl80. It's announced that their prices will be high, so it's assumed that honor will be inflated, items will be more expensive and lvl80 PvP gives more honor. However honor points still worth something so if you have nothing else to do or enjoy PvP, you can stockpile honor (beware of the 75K honor cap).
Tip: stockpile PvP tokens too. It's quite possible, that on lvl 80 there will be a new repeatable quest that provide inflated honor for PvP tokens. So you should not change the tokens into honor on 70.
3) Professions: the epic recipe you farmed so much / were so lucky to get, is soon worth nothing to anyone except twinks. So it is time to rethink what do you expect from your profession. There are two good, one debatable and several bad reasons to make your choice. If you made it, don't hesitate to perform the change, relevel your new profession now, when materials are widely available! The reasons:
- financial: how much money will I make from these professions? If you choose profession based on money, you will be rich in Northrend without much effort. Obviously the gathering professions make the money: herbalism, skinning and mining. And they are profitable while leveling, so if you want to make money in Northrend, drop your crafting profession and go out to pick flowers or ores, or skins!
- profession-specific buffs/abilities. If you are an alchemyst, the same elixir give you more buffs. If you are an enchanter, you can enchant your rings, while others don't. I've found a great list for helping this choice. Don't forget, that this list can become obsolate by the next patch. You can always find current information on WotlkWiki. These buffs will help you raiding or PvP in lvl80 endgame. However if you don't plan to participate in serious raiding or PvP, these differences are irrelevant. The daily quests on 80 won't need flasks on, just like the Isle of Quel'Danas quests.
- BoP items: every profession offers items to create what only you can use. These items will be huge boost in early 80. You can wear T7-T8 equivalent items while others are running around in lvl70 epics or lvl 75-80 blues. However don't forget that these items can cost a lot, getting the recipes may take more time than farming a good instance drop and these items will surely be replaced by later raid drops. So a choice based on BoP is only wise if you either swimming in money (so you can afford buy the materials and vendor the item a month later), or if you don't plan serious raiding (in this case BoP-crafted is the best PvE gear available to you)
- what I already have: keeping a profession because you already have it is a dumb reason and should only matter if you have two candidates with equal benefits depending on the previous reasons. The old recipes won't worth much on 80 and the slvl1-360 releveling costs less than 1000G if you do it smartly (for gathering it makes money during leveling). So if you keep a useless profession just because you have it is dumb.
- what the guild needs?: keeping alchemy because there are no other alchemists left in the guild is just dumb. Not only because your guild will (against all promises) disband or radically change while making it's way to 80, but also because you are not the sacrificial lamb of the guild. Unless they pay for your work, don't have a profession that does not benefit to you.
- profession only mounts: if you pick tailoring just to have a flying carpet, you are wise like a lvl 10 ogre. Period.
- what the other people of my class/specs choose? If you follow the flock, you are a sheep.
- /roll. I really knew someone who picked profession this way.