There isn't as much difference between negative- and positive-mass wormholes as between negative- and positive mass black holes. Negative-mass black holes have no horizons and a naked repulsive timelike singularity. A negative- (at infinity) mass wormhole would look basically like a regular wormhole. The local spacetime curvature would, of course, be different, but the topology would remain the same, S^2xRxR or similar.