change domain

I now have a domain name as a 6 year, now I would like to change domain name. Is this good for SEO?

Changing is almost never a good idea.

So say if I had a great domain name(this is not my domain or site, just an example) - boatforum.com - and I bought a site called - nothingbutboatsforum.com and that site was getting good traffic already. We all know that boatforum.com is a 1000 times better then nothingbutboatsforum.com and with that name change, the traffic would increase even more. Is there any way to keep the links that show in search engines to be redirected to the exact thread that link from google searches , but with the different domain name?

For example, someone searched on google "how to fix an out board motor" and this link is on google - nothingbutboatsforum.com/forum/How_to_fix_a_motor.html. Is there away of doing a 303 redirect from that link to - boatforum.com/forum/How_to_fix_a_motor.html?

Park the old on the good within your hosting panel, and your non-www redirect will take care of the rest.
[How to] Setup www.yourdomain.com or yourdomain.com (www vs. non-www) - With .htaccess 301 redirects - vBulletin SEO Forums

So you are saying that the link that is on google now(with or without the www., doesn't matter) will stay on google and eventually switch over to the boatforum.com with out dropping it?

There's no telling what will happen. It's pure speculation.

the 301 will inform of where it went. What google does with that information is entirely up to them.

Figured. So when I need to do this setup, follow the instructions you linked me. The google link should go to the new domain's post that was on the older domain?

yes, it will redirect by basically swapping out the .com's