Are there any special requirements when importing an encrypted wallet.dat file.
Also
If I've already downloaded the blockchain with a new wallet that has a zero ballance is there a way to update the wallet.dat that will allow it to work with the blockchain that's already downloaded or will I need to download the entire blockchain again.
Thanks
It will work automatically. All you need to do is replace the wallet.dat in the data directory with yours. If it doesn't show the correct balance then you will need to have Bitcoin Core rescan the blockchain, but this does not involve resyncing or redownloading anything. It is usually fairly quick. Normally you won't need to do that.