Effect of a longer loopback cable on radiated EMI
In short:
to meet compliance you will probably have to retest, I haven't read the standard myself (It's paywalled). But there are resources that deal with the change in compliance.

I'm not sure that many standards let you prove the compliance without testing, and yes it will make a difference on the cable length.

A word of advice: it would be better to find an EMC consultant or someone who knows the standards and has done the testing before because there might be a way to get the device classified under a different standard OR knows a way to get it grandfathered it to the new standard without testing. This has worked for me in the past.

This is a good resource if you really want to calculate and learn about antennas. The cable could be approximated as a coaxial antenna, but what would be hard to model is what voltage source for the antenna is. On paper it might be possible to run through the equations backwards for the length of cable you have using the radiated test data you have, but the length of the cable will change the resonant frequency. This will make it hard to determine what the radiated emissions of the longer cable would be from the original test data.