Yes.
In Nevada, postnuptial agreements (entered into after marriage) are very different from prenuptial agreements (entered into before marriage). Why? For starters, because different law applies. So resist the urge to simply change the word 'prenuptial' to 'postnuptial.'
Prenuptial agreements, which are actually called premarital agreements in Nevada law, are governed by the Premarital Agreements Act which is spelled out in NRS 123A. One key provision is NRS 123A.050, which allows parties to a premarital agreement to contract with respect to:
(a) The rights and obligations of each of the parties in any of the property of either or both of them whenever and wherever acquired or located;
(b) The right to buy, sell, use, transfer, exchange, abandon, lease, consume, expend, assign, create a security interest in, mortgage, encumber, dispose of, or otherwise manage and control property;
(c) The disposition of property upon separation, marital dissolution, death, or the occurrence or nonoccurrence of any other event;
(d) The modification or elimination of alimony or support or maintenance of a spouse;
(e) The making of a will, trust or other arrangement to carry out the provisions of the agreement;
(f) The ownership rights in and disposition of the death benefit from a life insurance policy;
(g) The choice of law governing the construction of the agreement; and
(h) Any other matter, including their personal rights and obligations, not in violation of public policy or a statute imposing a criminal penalty
Postnuptial agreements (also called postmarital agreements) are governed by NRS 123.080. NRS 123.080 only allows married couples to alter their legal relations as to property, although there is an exception for couples immediately separating:
"A married couple cannot by any contract with each other alter their legal relations except as to property, and except that they may agree to an immediate separation and may make provision for the support of either of them and of their children during such separation."
Comparing NRS 123A.050 to NRS NRS 123.080, it is clear that parties can do much more in a premarital agreement than in a postmarital agreement. Most importantly, couples cannot contract regarding alimony or spousal support, unless they are immediately separating.
A postmarital agreement that contains alimony provisions is likely void and unenforceable. See, for instance, Cord v. Neuhoff, 573 P.2d 1170, 1172, 94 Nev. 21, 24 (Nev. 1978). It is possible that a severability clause, which states that if a term is invalid the rest of the contract is still enforceable, might save the remainder of the agreement. But alimony provisions are such a large and important portion of such agreements that the entire agreement is still likely to be thrown out.
The Premarital Agreements Act also has specific requirements to be enforceable, laid out in NRS 123A.080 (1), which states:
A premarital agreement is not enforceable if the party against whom enforcement is sought proves that:
(a) That party did not execute the agreement voluntarily;
(b) The agreement was unconscionable when it was executed; or
(c) Before execution of the agreement, that party:
(1) Was not provided a fair and reasonable disclosure of the property or financial obligations of the other party;
(2) Did not voluntarily and expressly waive, in writing, any right to disclosure of the property or financial obligations of the other party beyond the disclosure provided; and
(3) Did not have, or reasonably could not have had, an adequate knowledge of the property or financial obligations of the other party.
Interpreting the language of NRS 123A.080(1) is a topic in itself. But be careful of the 'or' between the (a), (b) and (c), and the 'and' between (1), (2) and (3), the combination of which makes many premarital agreements unenforceable.
There is no statute for postmarital agreements corresponding to NRS 123A.080. Despite this, it seems prudent to meet at least the same standards in postmarital agreements as required in premarital agreements.
In addition to different law, postmarital agreements have do address the parties in a different situation. A married couple will likely have already acquired community property that needs to be divided, which is not an issue in premarital agreements. While premarital agreements often confirm property as the separate property of one party, postmarital agreements may need to transmute property from community property to separate property.
The difference between the applicable law and the situation mean that postnuptial agreements should be drafted separately from premarital agreements. You cannot just change the word 'pre' to 'post' in a prenuptial agreement and think everything will be okay.