Shane hit the nail exactly on the head.
The new rule is that if your current skills could all be had at the same or cheaper build expenditure (or if you have the free build to make up the difference), you can choose to change your class. This then changes the amount future skills cost you, your body total, max armor, etc. No skills are lost (this is where Spirit Forge comes in, but don't be shocked if it changes in some way because of this rule). This is NOT mandatory (for example, a low level fighter with read/write, first aid, and healing arts might technically have cheaper skills as a templar, but if you want to keep the extra body points, you can stay fighter). There are further caveats in the book about when/how this can be done so as to not drive your logistics people insane, but that's it in a nutshell.
Overall, the intention is to allow for more "organic" skill-selection instead of locking a character into a path chosen the first time he/she is played, as well as to help some with the cross-class slump that tends to happen in the mid-levels where some players feel like templar or adept were the worst possible choice they could have made.
~JP