I've been a while without tracking the coin, I only have two doubts.
1) Will a new coin be posted on this hardfork (if so, which one)?
2) About Hardfork if I have coins in Bittrex can I get the other currency?
This is a miss-conception probably due to the recent cases of Bitcoin forks aimed to create new coins (Bitcoin Cash, Gold and so).
A hardfork is just a change in the very deep rules that govern a blockchain (called the consensus set of rules) that makes old versions of the clients to become incompatible with the new mined blocks. In this case, Sia is changing the algorithm that adjusts the mining Difficulty. The new version was introduced 6 months ago, and in that version this hardfork at block 135000 was already programmed.
If a hardfork becomes contentious, and so one part of the community accepts the new rules but not the other, 2 coins emerge from the hardfork (as with Bitcoin forks)
In the case of this fork of Sia, all the community is accepting the change. Almost all the hosts are using version 1.3.0 (the compatible version), all the exchanges, all the mining pools... This hardfork is being accepted by all the parts, and so there will be no creation of a new coin. After block 135000 all the coins in circulation, mined or in exchanges will be part of the upgraded blockchain. The legacy blockchain, together with the old version of Siacoins, will be abandoned, as no one is supporting the legacy coin.
In summary, the legacy coin is going to disappear, and your exchange will issue you the same amount of coins you had in the new coin. Of course, the name of the coin will remain as "Siacoin"
Do you need more info? Check the official post about the fork: https://blog.sia.tech/reminder-upcoming-sia-hardfork-e0ffcde9ebcc