How hard is it to obtain a TS (SCI) clearance if you currently hold a secret?

It's not likely to speed up the process although you'll have a lot of your references in there. I was put down as a reference for a soldier in my Reserve unit they were upgrading the clearance on and I'm pretty sure it still took the investigators about 8 months to get to me for an interview.

Your S2 or security office type people will usually take care of the initiation of the paperwork. It will be best on your part to start getting together all the people you need references from for the past 10 years for a TS investigation and filling out a copy of an SF-86 on paper. This way, when you need to fill in the electronic version, you will have it all ready to go as the window to enter the information once it opens is usually only a week and you don't want to be scrambling to get current information for everyone in order for them to be contacted at the last minute.

It should make your SF86 easier to fill out. That's about it. You will have everything already in e-QIP (depending on when you got your secret). You just have to go back 10 years on a TS. Which should only add 3 years to what you have on your secret.

It takes time for the investigators to get through things when you apply for a TS. I think I had my interview a few months after I submitted my e-QIP. Then they will go talk to definitely the "people who know you well" section of references. They might talk to anyone listed as a reference honestly. I think I submitted my PR in May last year and the investigation was closed out in January this year. That's for a renewal. I was in AIT when I had my interview for my initial TS - so five months after I submitted it at MEPS and reception. I can't remember how long it took to close. But you're looking at least 8 months.

I did see when I was S2 two S3 officers had their investigations closed out in 3 months - but we were deploying. I think they finished it right after we deployed.