As long as you meet the requirements of that agency`s particular certification level AND they are under the RSTC, then there should be no problem crossing over. For example: if you are Open Water certified by PADI, then SSI, NAUI, SDI, YMCA, and IDEA should all accept your certification for crossover to their particular company since they are all under the same standards. You will have to cover the card and processing fee.
There are exceptions to this rule. If you are trying to cross over on an advanced level or professional level, some of the agencies may require more specialties, experience, or other courses to complete a crossover. Professional crossover requires a full blown crossover course at all professional levels (DM to DiveCon, SSI OWI to PADI OWI, etc.), regardless of the agency you are coming from.
For a current list of who is part of the RSTC or WRSTC, please visit: http://www.wrstc.com/main.php
Simply click on the appropriate region to see council members.