You can use your .htaccess files to allow visitors to view your site with your old domain name instead of the new one. This maintains the original SEO you did with the old domain but it may have lost integrity over time so you may have to work at it again. If your old domain did better by you, however, I recommend having it available as well as the new one. If you point all traffic to one location, pick the one that works best for your current traffic and intention.
If your new domain gets crappy traffic, point that traffic to the old domain in the .htaccess file. But only build on one domain, don't try and build them both.