It is unclear given the description above whether you are talking about a single entity or two distinct entities with regard to the contract and your employment. Specifically, it would help to know what you mean by "partner company."
Generally speaking, under California law a company can transfer an employee or establish conditions on employment however it chooses. In other words, a company can transfer an employee without his or her consent to another location. Indeed, a company can terminate the employment relationship for any reason it likes (other than an unlawful reason like race or age discrimination or harassment, etc.). If you are talking about two separate companies (i.e. two separate employers) then the most one can do is terminate you. If the other (the "original company" that you prefer) wants to keep you, however, then it, and you, are free to make that arrangement. This response assumes that no employment contract or union agreement exists, either of which can greatly change the analysis.
This response is intended to provide only general information on the topic presented by the question above, and not to be treated as legal advice. No attorney client relationship is intended to be created by this posting.
Based only on the limited information provided, the company's transfer appears perfectly legal. Unfortunately, employees and job applicants have very few employment rights, and employers have a lot of leeway in how they choose to run their businesses. In general, an employer can be unfair, obnoxious or bad at management. And an employer can make decisions based on faulty or inaccurate information. An employer has no obligation to warn an employee that he or she is not performing as the employer wants. It’s not a level playing field. An employer hires employees to provide work for its benefit, not for the benefit of the employees. Don't expect the employer to take care of its employees; it doesn’t have to and it rarely does.
There are some limitations on what an employer can do, mostly in the areas of public policy (such as discrimination law or whistle blowing), contract law, union-employer labor relations, and constitutional due process for government employees. Please see my guide to at-will employment in California which should help you understand employment rights: http://www.avvo.com/legal-guides/ugc/an-overview-of-at-will-employment-all-states. After you take a look at the guide, you may be able to identify actions or behavior that fits one of the categories that allows for legal action. If so, an experienced plaintiffs employment attorney may be helpful.
Employment rights come from the state and federal legislatures. One of the best things people can do to improve their employment rights is vote for candidates with a good record on pro-employee, anti-corporate legislation. Another way to protect employment rights is to form or affiliate with a union, or participate in a union already in place.
I hope you can resolve your situation and wish you the best.
@MikaSpencer * * * twitter.com/MikaSpencer * * * PLEASE READ: All legal actions have time limits, called statutes of limitation. If you miss the deadline for filing your claim, you will lose the opportunity to pursue your case. Please consult with an experienced employment attorney as soon as possible to better preserve your rights. * * * Marilynn Mika Spencer provides information on Avvo as a service to the public, primarily when general information may be of assistance. Avvo is not an appropriate forum for an in-depth response or a detailed analysis. These comments are for information only and should not be considered legal advice. Legal advice must pertain to specific, detailed facts. * * * No attorney-client relationship is created based on this information exchange. * * * Marilynn Mika Spencer is licensed to practice law before all state and federal courts in California and can appear before administrative agencies throughout the country. She is eligible to represent clients in other states on a pro hac vice basis.