It’s possible, if you are several releases (years) behind, to end up with a DragonFly system that can’t compile and install the current release, due to incremental changes over time.  It’s rare, but it could happen now between, say, version 3.4 and 4.0.  The usual solution would be to incrementally upgrade in order, which is a lot of building and updating.  The alternative is the new installworld-force option from Matthew Dillon that forces a new set of binaries into place.  Use as a last resort.