How to run new .meas commands without having to rerun the whole ltspice simulation?
I've never used it, so I cannot give more details.

After the run, if your plot window is active, there is a point available:

File -> Execute .MEAS script

From the LTSpice help:


  .MEAS statements are done in post processing after the simulation is completed. This allows you to write a script of .MEAS statements and execute them on a dataset. To do this, make the waveform window the active window and execute menu command File=>Execute .MEAS Script. Another consequence of .MEAS statements being done in post processing after the simulation is that the accuracy of the .MEAS statement output is limited by the accuracy of the waveform data after compression. You may want to adjust the compression settings for more precise .MEAS statement output.


So you are correct, the data is all there and the statement is just performed on the available data set and nothing special happens during the simulation.