Can I cancel my old work permit and obtain a new one without leaving Thailand if I have documents from a new employer?

In that case maybe you can ask to immigration office. I'm not sure, but you may be able to cancel work permit and extension and apply extension with valid documents from new employer, not like just extension like most of the cases lack of documents. I haven't read any report here, I heard personally one case that someone could switch employers without leaving when old and new employers working closely and accordingly. Still it's a while back.

I guess it's worth to ask Immigration and MOL if you need.

Sandeep Sandy, There are no free and automatic 7 days granted when you cancel extension.