I don't consider it a fair swap. The ability to search for traps (and get a limited bonus on ref saving throws against traps) is a liability, I can imagine almost any rogue being glad to not get that ability! Any normal rogue that doesn't put skill points into search effectively doesn't have it anyway.
For something as useful as Bardic Knowledge I'd be looking for a more significant swap - perhaps losing uncanny dodge or evasion, or halving his sneak attack progression.