There's probably something in Google if you look, but realistically there are only two options:
a) Move data off the disk, change the filesystem type, move the data back.
b) Shrink the NTFS partition, create an EXT4, start to shuttle the data from one to the other and progressively shrink one, expand the other, until all the data is moved, then delete the NTFS partition and expand EXT4 to 100%.
Both will be rather slow .. depending on how full the disk(s) are.
NB: No idea if there's an actual conversion routine. I wouldn't normally trust that kind of thing.
Ok, I suspected that's what I have to do. Thanks for all help