Re-analyzed my dissertation work, how should I explain this in a cover letter to a journal editor? I graduated recently, and I am trying to get some journal publications out of my dissertation.
Unfortunately, I just found out that changing inclusion/exclusion criteria and statistical analysis would improve the quality of my study. So I re-analyzed the data with the revised methods and statistical analysis.

The number of included subjects decreased a bit. The results did not change dramatically, and qualitatively the same.

My question is...now how should I explain this to a journal editor in a cover letter?  The numbers in the results won't be the same as the ones in my dissertation.  The methods and statistical analyses are not exactly the same, but overall pretty similar between before and after making the change.  

I am still considering that, in the cover letter, maybe I should mention that the work is based on my dissertation, but I made improvements upon the submitted dissertation.  Would this be acceptable to a journal editor?  If not, how should I handle this situation?  

Everyone's advice would be appreciated.
Your submission to the journal will be a separate paper to your dissertation, so it is not a problem if you revise the method and get slightly different results.  Just undertake the analysis with your new method, and write that analysis up in its own right in the new paper --- the journal referee will assess this work on its own merits.  Normally in this case you would cite your dissertation as a precursor to the work in your present paper, but note that your analysis differs from the analysis in that work.  (Perhaps explain this briefly in a footnote.)