Unless the reason for migrating to the new version is to get a game-ruining bug sorted, I'd consider sticking with keeping an directory of the old version within which to run your old game, whether or not you also download and put into a new directory the newer version in order to play with whatever new 'toys' (in the forthcoming example, minecarts) are being rolled out in that.
I've previously (40d times) copied older saves into newer-versioned program areas, when it was largely safe to do so, but especially where the key new features are reliant upon (if not entirely consisting of) improvements to the Raw-files you're effectively neutering the new-version with your old-version saves. Unless you know what you're doing in integrating the base raws of the new into the old (bearing in mind you'd have to tweak some unused functionality in an already-running world), and that could go wrong.
Given how much space the game (and its saves) takes up, I've ever really batted an eyelid at having multiple folders named "df_31_19", etc (i.e. the default main folder names one gets when extracting the .zips) in the same area, and even if (and I rarely do these days) I go to the trouble of putting a .lnk in some location (Programs menus, Quicklaunch area, Desktop, directly in My Documents, whatever), I'd usually qualify each with an equivalent name as well, so multiple version-links side-by-side wouldn't be a problem.
But, really, the most relevant part of my reply was in the first paragraph, you probably didn't need to read the rest, I suspect.