I also had to migrate a big C2 project to C3, and because I used an old version of SpriteFont Generator, I didn't have spacing data in new format. So I created this small conversion tool, maybe it will be useful for someone else:
dropbox.com/s/d5y7v7wskkabuxq/SpriteFont_convert_spacingdata.capx
It converts from this:
{""c2array"":true,""size"":[2,13,1],""data"":[[[24],[34],[41],[43],[51],[52],[53],[54],[55],[57],[58],[64],[66]],[["".,""],[""-""],[""1""],[""+""],[""2""],[""7""],[""3""],[""5""],[""49""],[""8""],[""6""],[""0""],[""x""]]]}
to this:
[[24,".,"],[34,"-"],[41,"1"],[43,"+"],[51,"2"],[52,"7"],[53,"3"],[54,"5"],[55,"49"],[57,"8"],[58,"6"],[64,"0"],[66,"x"]]