Orchard still support "old school" aspx / ascx templates alongside Razor. You can see this particularly when it can't find a certain partial view, it displays a list of all the alternate filenames it looked for and it always includes .ascx versions as well
as .cshtml and .vbhtml.
Additionally if you just omit the [Themed] attribute from your Controller / Actions and return a normal View(...) result instead of a ShapeResult then all the Orchard templating bits should get completely bypassed.
So I don't think there's really any upgrading to do. When you add new views you can use Razor (and believe me it's worth learning it - there are things you can do with it that are simply not possible in aspx; for instance template delegates which are like
mini template functions - quite apart from it just looking and feeling way nicer) - but all your aspx/ascx bits will continue working as normal.