A great deal depends on just how unethical you wish to be.
If you quit your present company, but let all your customers know that you are now going to be at XX company, that's perfectly ok. You've done nothing wrong and you are perfectly entitled to sell to said customers if they choose to look you up at your new company without you going to them first.
If, instead, you negotiate with your customers to migrate to this other company even as you are working in your present one - you open yourself up to big time lawsuits, and this WILL come out in discovery. As an apparent COO or actual COO, you have fiduciary duty to your present company and that includes not actively working against it.
Of course, this assumes the present company doesn't sue when it discovers all/most of its customers are migrating to somewhere else. This is a pretty safe assumption if in fact you are able to perform the migration.
There is a range of actions which lie between these extremes as well.