how would a MECIR upgrade differ now, compared to if I was to let my SECIR expire and do it after that?
If you keep the single rating current, you only need to demonstrate an asymmetric missed approach, asymmetric circling and EFATO in the multi to upgrade the rating. You would not need to do all the approaches again (but you would obvioulsy need to do 2 approaches to get the asy missed approach and asy circling). If you have no rating and haven't for 12 months, you will need to do the initial issue flight test whether single or multi.
IREX covers single and multi ratings and the theory credit is valid indefinitely.
If you're flying in PVT ops, the PIFR is the way to go. There is no upgrade path from PIFR to CIR but the PIFR has no renewal nor recency. I think there is a AFR style check for the PIFR every 2 years.