If Warren were to vacate her seat, Massachusetts law says that a date of a special election "shall not be more than 160 nor less than 145 days after the date that a vacancy is created or a failure to choose occurs" -- but in the meantime, Baker has the authority to appoint a Republican successor.
However, Reid and his advisors have found an awkward work around: Warren can file an intent-to-resign letter 145 days prior to a January 20th inauguration date, which would block Baker from making an appointment as Warren would still be in office. However, in this scenario, if a Clinton-Warren ticket were to lose in November, Warren would have to rescind her resignation and run for what would then be an open seat.
Brian McNiff, a spokesman for Massachusetts Secretary of State William Galvin's office, confirmed to CBS News the legality of Reid's loophole.
Alternatively, and perhaps more realistically, Warren could also file for resignation on November 8th, the day after the election, in the case that Clinton wins, giving Baker the authority to appoint a temporary replacement who would serve between Clinton's inauguration and the date of the special election. This would potentially eliminate the possibility of a Democratic majority come January, while still giving Democrats the opportunity to quickly retake the seat.
http://www.cbsnews.com/news/if-warren-is-clintons-vp-harry-reid-has-a-plan-to-replace-her/