I currently have a secret clearance and my employer wants me to obtain a TS. I plan on quitting in the next 5-6 months which I know isn't nearly long enough to complete the process. My question is once I start the investigation and quit could this potentially cause any issues with my current secret clearance?
The only thing I can think of is if they start the TS investigation, you quite and find a new job but for whatever reason they can't accept your current secret clearance and have to put you in for a new investigation... which could get blocked by the "ongoing" TS investigation.
If you get to the stage where a new employer wants to make you an offer, they should be able to determine if they can accept your current secret clearance.
If you are undergoing an investigation and leave the job before the investigation is completed the FSO is supposed to notify OPM to discontinue the case. It has no impact on current eligibility at a lower level, however, if you are no longer in a position requiring the Secret it will be withdrawn. If you are employed in a similar position within 24 months and the last investigation is within scope then they can reinstate eligibility.