You can do 301 redirects which will transfer much of the google juice from the old to the new site, however you will likely not have the domain authority on the new domain which will reduce the rankings a bit - although this will come back over time.

Why not put new content on the old domsin?

You can do 301 redirects which will transfer much of the google juice from the old to the new site, however you will likely not have the domain authority on the new domain which will reduce the rankings a bit - although this will come back over time.

Why not put new content on the old domain?