Postal to priority & issuance of 2 COS for same applicant

Hi everyone

My current employer has issued me with a COS that I used to send my tier 2 general visa application (extension) by post, however I have received a better job offer and the new company is willing to issue me with a new COS.

I would like to know how soon the new company can issue me with the COS as I heard that it is not possible to issue another COS until you have used the first one. What does "used" mean? Is the COS considered to be used from the time your application is received by UKVI or is it from the time they give you a decision?

I would also like to know if it is possible to turn my original postal application to a same day or priority one (I have already given my biometrics).

Thank you for your contributions.