Take a look here – https://appfinite.com/topic/legacy-1-3-for-genesis-2-0-html5-is-now-available/
There shouldn’t be any issues or anything unless you are mixing code from the old version in with the new version. If you’re going to keep some parts of the old version then you’ll need to take a look at the link above. The link above points to other links/tutorials on how to manually convert an older Genesis theme to the latest Genesis 2.0