str vs bytes isn't as bad as people make it out to be. You'd be used to it within a week. The other stuff is mostly just semantic changes like range vs xrange or dict.items() returning an iterator now vs a list.
At this point, the bigger issue might be learning all of the new features people were missing out on by not moving earlier, like asyncio.
If you have a large python2 project with 100 dependencies, will all of those have python3 versions? And will they be compatible with each other in the ways your current dependencies are?
[1] A one that I often seen was MySQL-python. That package hasn't been touched since 2014. Another one I've seen was PIL, that one was last released in 2009, and isn't even on PyPI, because it is that old, fortunately maybe because it's not on PyPI, people are more aware of Pillow which is a drop in replacement
It's not easy to determine which versions of the library to upgrade to however. What's the latest version compatible with both python 2.7 and 3.7. Would be nice if pip had a setting to get that.