A Will prepared in one state would be valid in another state, assuming it was valid in the orginal state, in your case, California. But as each state has its own laws, and some unique probate procedures, you'd probably be better off (assuming there is no issue about mental competence or undue influence) getting a new Will prepared in Nevada by a Nevada lawyer.
Better still, I'd suggest setting up a living trust, transferring substantially all your assets to it, and naming your partner as the (sole or principal) beneficiary of the trust and/or any other one else you'd like. You see with a Will it must be filed for probate, your statutory heirs must be notified, and they could contest the Will or delay it. It's much harder to derail a Trust. You'd want to have a pour-over Will too for anything left out, and be sure to change all the benficiaries on insurance policies, 401(k) plans, IRA's etc. Read Estate Planning, Wills, Trusts and Probate on FreeAdvice.com and get a lawyer in NV from AttorneyPages.com -- there are some really good ones listed.
