It totally depends on your BLOG NICHE, domain name, and Domain PR.
Suppose, If you are posting about blogger platform and your old domain name is mysite.com and your new domain is mybloggersite.com then new is better then old (Only when then your old domain have not PR).
And if your old domain is mybloggersite.com and now you want to change it to mysite.com then it is not good for SEO.
You should choose domain name related to your blog niche.
If your domain name is old 1 or 2 years, then don't change otherwise it will be dangerous.
Hope you understand.