Can a Barbarian2/Warlock3 retrain an invocation into Pact Blade?

@kilpatdsCan a Barb2/Lock3 retrain an invocation into Pact Blade? (ie: is 5th level 5th level? or 5th warlock level? also, retrain rules) up to the DM – depends on group expectations

— Mike Mearls (@mikemearls) August 22, 2014