On Wednesday, December 4, 2019 at 3:48:30 PM UTC-8, wrote:
On Wednesday, December 4, 2019 at 5:39:04 PM UTC-5, wrote:
On Wednesday, December 4, 2019 at 2:46:39 PM UTC-7, wrote:
On Wednesday, December 4, 2019 at 4:28:18 PM UTC-5, wrote:

My question is about doing a FIRC early, not late. Many months before the expiration date.

There are circumstance that allow this, but only up to about 3 months prior to expiration. Advisory Circular 61-65H discusses this in section 30.2..

Yup, but am I stuck with the time of year that I happened to first get the CFI, or can I shift it to some otherwise desirable soaring-relevant FIRC? This rather long-winded section from that AC seems to say it CAN be done early - but will be good for 24 months from that time. Same as an early BFR (or added rating) resets the BFR calendar.

"30.2.1 Renewal Outside of the 3 Calendar-Month Period. If the flight instructor certification renewal action occurs outside the 3 calendar-month period of the expiration month on the flight instructor certificate, the renewed flight instructor certificate will be valid for 24 calendar-months from the month the renewal requirements of § 61.197 were accomplished. The applicant must present a FIRC graduation certificate to a FSDO within 3 calendar-months of completion of the FIRC. The renewal action of § 61.197(a) must be accomplished before the expiration date shown on the applicant’s flight instructor certificate. If the applicant completes a FIRC more than 3 calendar-months prior to the expiration month of the current flight instructor certificate, the expiration date will be 24 calendar-months after the month the FAA Form 8710-1 application is submitted to the FAA, provided the application is submitted within 3 calendar-months of completion of the FIRC."

- so theoretically one could do a FIRC up to almost 6 months before the CFI expiration, submit it 3 months later - within 3 months of the expiration - and retain the original expiration month, albeit that "submission" may be difficult (travel to a FSDO?). Or one can do it any time, but lose part of the original 2 years. Is that correct?

No.
If you complete the FIRC on Jan 5, then you can wait until Apr 4 (not 5!!) to present the certificate to the FSDO. At that time your CFI will be current until Apr 30 2 years later.

Today, the SSF folks can do this for you at the end of the FIRC so you don't need to go to the FSDO.

Many years ago, I lost track of the drop dead date and remembered while driving to work on the last possible day. Had to go home to get the certificate then drive a fairly long distance to get to the FSDO. The inspector pulled out a calendar and we both counted the days to confirm that I made it by an hour or so - since I got there just before closing.

I also highly recommend the SSF FIRC over any other options, especially if you are a CFIG only.

Tom