ESB has the least changes because its the most well made. It relied on human actors and not effects. So unless Lucas creates a CGI harrison ford or replaces the yoda puppet theres not much more he can do to "improve" it.

And i have a feeling Marquand would be fine with the musical number, especially since he already was shooting a musical number in Jedi to begin with (it was edited down because the technology was thankfully not yet advanced to allow filmmakers to produce such crap). Marquand was what we calla puppet director. ROTJ was always more Lucas' film than Marquand.