I would assume you need to recreate the mel spectra, but feel free to create an issue with this question in the repository.
EDIT:
I just talked to Grzegorz (author of the repo), who explained, that prepare_mels.sh allows you to load mels directly from you dist instead of processing wav files on the fly and is therefore recommended.

With mels on disk use --load-mel-from-disk --training-files=filelists/ljs_mel_text_train_filelist.txt --validation-files=filelists/ljs_mel_text_val_filelist.txt
The filelists specify the paths inside the dataset, just check filelists/ljs_mel_text_val_filelist.txt as an example