1. If you are currently re-downloading the client, and are in initial "downloading" phase (around 11GB), you can exit the launcher to interrupt this download and play 1.2 tomorrow without having to complete it. If you reach any "installing" phase, you will need to complete the re-download.
Last night I was on 11.1GB remaining of the original ~11.8GB client redownload, and as instructed in the sticky, part of which is quoted above, I stopped the download and closed the client.
Now that 1.2 is downloadable, the launcher is continuing to download the remaining 11.1GB.
The reason I chose to not redownload the client is that I would be downloading for ~30 hours at my internet's download speed. Now it looks like I cannot play 1.2 for another 30 hours from now?
Note: I definitely did NOT reach an installing phase for the ~12GB client redownload.