Can I update to the 15.0 wallet and NOT have it re-download the whole blockchain again?
Yes, you can. The normal update process of just installing Bitcoin Core 0.15.0.1 on top of Bitcoin Core 0.13 (i.e. overwriting the previously installed version with new binaries) does not destroy any blockchain data. Even uninstalling Bitcoin Core 0.13 does not delete any blockchain or wallet data. All of that is stored separately from the installation itself. So yes, you can just download and install Bitcoin Core 0.15 and not have to redownload the entire blockchain. It will, however, take a while to start up with Bitcoin Core 0.15, especially if you were using Bitcoin Core 0.13.0. This is because it both needs to update the database structure (this changed in 0.15) and, if you were using 0.13.0, it will need to redownload the blocks that were found after segwit's activation so that it has them in the segwit format.
The Bitcoin blockchain seriously needs to be clipped off. Take off the first 470,000 or so blocks and begin from there, no one has enough hash power to mine their own new blockchain with a 51% attack anymore. Downloading most of that 145GB blockchain is simply ridiculous.
Doing that violates Bitcoin Core's security model and there are so many things wrong with doing that that can be attacked. Clearly you have not thought through ramifications of doing so or you do not understand the ramifications of doing that.
