Even after you win custody, your spouse can challenge it. If there's good reason to shift custody back to the other parent, you can lose custody of your children, or have your arrangement substantially altered.
These are among the most common reasons Courts consider changing custody: a) when a custodial parent attempts to alienate a child's affections for the other parent, or blocks access or visitation; b) when changes in employment entail a more demanding work schedule, and potentially compromise time and attention devoted to parenting.
With employment changes, there's a higher probability of a custody modification if the new position involves increased travel or commuting, or requires change of residence. Your vulnerability increases if your spouse has a more flexible work schedule than you.
If you intend to get involved in another relationship, having your new love interest stay overnight in the presence of your children is another choice to consider carefully. A conservative judge may view this as creating an unhealthy environment for your kids. It's wiser to have an overnight guest when your children are staying with your spouse.
Any action that can be interpreted as ignoring the needs of your children may be used as a legal weapon against you. Before you act, think about how a judge might look upon your choices.