I just reconverted everything with only one minor hitch.  I offer some statistics for those that might consider doing this sort of thing.  My Dimension Pro Multisamples folder contains only what came with X2 Producer (presumably the same as if you bought DimPro separately) plus a few free content packs from Cakewalk since last November.

There are several DimPro multisample folders that already use wav files, so they can be skipped.  These are :

05 - Guitars
      - only Godin Artisan, other guitars use flac files
32 - Rapture Scapes
34 - Twisted Textures
36 - Twisted Modeling
40 - Stabs and Hits
96 - Virtual B3
99 - Vintage Sound Library
Cabinet and Amp Simulations
Digital Sound Factory E-MU Proteus Examples
Digital Sound Factory Keys
Digital Sound Factory Symphonic Strings HD
Hollywood Edge FX
Pro-Sounds

The rest of the multisample folders contain flac files.  My installation has 12,873 flac files.  These were converted in less than five minutes in one go using FLAC Frontend.  Just select all the folders you want, drag them into FLAC Frontend, check "Keep foreign metadata" then hit "Decode".

The only oddity I encounted was that FLAC Frontend refused to convert the Platinum Samples Rock_Legends freebie (which X2 Producer owners got a little while back) since the flac files "contained no metadata".  I don't see why this should cause a failure but just unchecking the foreign metadata box fixed things.

For the sfz files, grepWin searched 19,133 files and replaced 27,054 instances of ".flac" with ".wav" in 560 files.

It took longer to type this message than to convert all the files...
 
Edit :  I then reconverted the Session Drummer 3 multisamples in case the samples had any metadata that needed to follow the conversion (I presume that there aren't any loops, but have no idea if there's anything else of importance).  As mentioned, the sfz files already contain references to wav files (even though the actual samples are flac files) so they didn't require updating.