The federal HIPAA protections allow only a 63 day gap (days of no coverage) between old and new health coverage. If you have a pre-existing condition, this means you will need to retain your previous coverage while transitioning to new coverage so that there is not a break in coverage of more than 63 days. If you don't, and your new plan imposes pre-existing condition exclusionary periods, you will be subject to them. In California, if your new coverage is with a fully-insured plan, your prior coverage was from an employer, and you lost that coverage because your employment ended, or because your employer stopped offering or contributing to health coverage, you are allowed to have up to 180 days without coverage.
An open enrollment period is the annual period of time that you are allowed to make changes to your benefits. During this time period you are allowed to change, add or delete coverage.