Why did "Dragon Ball" (the original TV series) change from being a comedy/ecchi, to a drama? I just wondered.

It probably is because of the typical shounen power creep, where the main character grows more powerful, so his opponents have to as well. Threats inevitably become more serious, so the tone has to follow suit.
Another example is Hunter x Hunter, where it begins as a light hearted adventure, then quickly turns into borderline horror, where children get their brain eaten by bug monsters.

The original series followed to some degree the legend of the “monkey king,” with some level of “naughty” or bathroom humor. When later episodes went to a more fight focused story line, the humor was phased out. That, and American audiences didn’t take to well to the “humor.”
-with help, thanks, and apologies, if required, to “The Complete Anime Guide” 2nd ed. by Trish Ledoux and Doug Ranney, Tiger Mountain Press, copyright 1997, page 37.