what is the different between the BitcoinXT fork and a altcoin?
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
should i worry about my btc in my papaerwallets?
BitcoinXT is an independent implementation of the reference bitcoin software ("Bitcoin Core") with a small change in the bitcoin protocol, specifically in the rules that define when a block is valid. Instead of limiting the block size to 1 MB, BitcoinXT says that blocks up to 8 MB are OK. (Currently blocks are ~0.450 MB each on average; but traffic is increasing the network will become congested when it reaches ~0.700 MB/block, which is expected to happen in 6--12 months time.)
That BitcoinXT change is programmed to become effective only in 2016. Until then, BitcoinCore and BitcoinXT will do the same thing, on the same blockchain. Clients, miners, and relay nodes can use either version, indifferently. After the criticla block, when the change becomes effective, things may get complicated.
If enough miners approve the increase in the block size limit, it is very likely that everybody will switch to BitcoinXT before the critical block. If too few miners approve, it is almost certain that everybody will continue using BitcoinCore, or switch back to it. Either way, clients should check, by the end of 2015, which version (Core or XT) has got more mining power, and make sure that their wallet is compatible with that version. Then they will not even notice the fork.
The possible complication is if the community gets stuck in a half-way state, with almost equal mining power on each side. Then there may be two blockchains, lots of orphaned blocks and branches, etc..
You don't want to know about that possibility; just pray that it does not happen, and, if it does, just stop issuing transaction until the impasse gets resolved as above. Even after the critical block, all the unspent coins that were created before that block will remain there, no matter what happens.