Personally I mostly fear the following scenario, which anyone can do at home (if he has a billion dollars lying around):
1. Short lots of bitcoins.
2. Build a huge mining cluster and completely mess up the block chain.
3. Watch prices drop as panic ensues.
4. Profit.
5. Lather, rinse, repeat.
You have to explain what you mean by 2.
For starters, he can create a new branch with none of the transactions from the last 1000 blocks. Or he can bypass X% of blocks as per your original post, which is not very dangerous for merchants but will piss off miners who wish to be rewarded for the blocks they find. And so on, he can get creative.
It can be done but it require 1)a backup of the old blockchain 2)enough miners that know what to do
Bitcoin never deletes blocks, so everyone will still have copies. The pools can be updated in no time, and they represent almost all of the network's hashing power.
Assuming a consensus can be reached not only about which chain is the real one, but also that discarding the computationally longest chain in some circumstances is desirable.
To reverse a 1000 blocks and to catch up then to make the longest block chain takes way more than 50 %, if you want to get it done in a decade.