I've licensed this skin, and have reviewed the differences between the skin-supplied and the core-supplied Default.aspx.  For those running 4.9, there are no appreciable changes and the replacement step is unnecessary (and, for the reasons that Chris outlined, ill-advised).  For those running a prior DNN version, the skin-supplied file essentially back-ports some of the attributes added in subsequent DNN releases (an enctype attribute on the dnn:form element, for example).  I believe this is to bring the skin into full W3C compliance.

For those running a prior DNN release, I suggest hand-porting the relevant attributes into your Default.aspx file until such a time as you upgrade to 4.9+.  For those running 4.9, no replacement is necessary.

Note that the above analysis was valid at the time of posting; nothing prevents the skin from being released with subsequent changes.  Reasonable caution is warranted.

I should also take the opportunity to add that this is a spectacular skin!

Brandon