Converting VS 2010 project to VS 2013

This is 5 year old project I am working on at work. The first thing I was prompted with when opening it in 2013 was to download windows SDK 7.1 to compile it in v100. ?
Then after I upgraded solution. I was prompted to change character set from multi-byte to Unicode.
THen...

Rebuild resulted in 3612 Errors, most of them stating "cannot convert.......". I don't know what the file count was but it might be 20-50 files.

What is this about and will I have to make so many changes to get this to build in 2013?