Before you start, think carefully about which version of FPOS you wish to install, taking into account the fact that with Android 13 there are issues such as the reduced security accreditation of the fingerprint sensor, I’m sure you’ve read all about that.
I haven’t encountered all of these by any means.
As far as I know there’s no intrinsic obligation to re-lock the bootloader.
If you have any other questions, don’t forget to say which version of LineageOS you’re using at the moment. It might be useful to say why you’re moving, so people can orient their replies usefully.