About 2 years ago i moved a site from html based to a new one (cms based) on the same domain and lost most of the rankings in SEarch Engines, lost a lot of visitors and 50% down on earnings from this website.

After 2 years i still have problem with rankings on Google for some significant keywords.

So it is better to go with a new domain, link from the old one to the new one, try to avoid content duplicates and when you are sure can 301 redirect to the new one.