I recently did a LED swap on my 01 SR5 and it went great for the most part other than the tachometer doesnt seem to work correctly anymore. Normally it would idle around 700-900 but now it wont move until I get to 15-25 mph or rev it when parked. I unfortunately got the tach needle snagged on the rag when doing a wipe down on the cluster and it was pulled over the resting peg. So that probably factors more than the LEDs. The vehicle runs and drives the same as it did before the swap and I disconnected the battery for about 30 minutes to reset the computer. So now I was wondering if I broke something in the tach or if there was a way to recalibrate it to true "0" RPM.

If I end up needing to replace the whole cluster due to this, how does one change the odometer to match the milage of the one one?

Can you not replace just the tach?
There is no "calibration" other than removing the needle and repositioning it.