I was filming in a concert, remotely (as I'm controlling another camera), but forgot to stop and re-start the Osmo on the 30-minute restriction.  Therefore, I lost a few songs on the Osmo before I recognised that it's stopped recording (again as I'm controlling another camera and hopefully I've got footage on this main camera).

Is there any method that I can revert to the previous firmware version without the 30-minute restriction, so that I wouldn't need to care so much next time?

Hello.
Sorry for any inconvenience.
It's not available to downgrade the Osmo firmware from the latest firmware version v1.6.2.10.

Thanks.

If you're going to be filming a concert in its entirety, you're probably going to have to rely on a standard video camera anyway -- unless you accept that you will lose just under 2 seconds of sound/vision every so often, when it starts recording a new file with the Osmo (there's a file limit of 4.2GB).

It's good for B-roll, but not for anything that requires uninterrupted recording over a long period.