Dgraph has no way of knowing if you have replaced the p directory. Dgraph would just assume that there’s no data. If you want to remove a bad alpha, you should use the api, removeNode, to remove that alpha, and then insert a new one. If you go this route, data would automatically be sychronised.
But, if you don’t want to do all that, and want a quick and dirty solution, you could rebuild the database and it would be quicker than this. All you need to do is delete the p, w directories from all the alphas, and put your desired new p directory (could be from one of the “good” alphas).