Hi Yves,
I know the option you say me, but my doubt is if I can do that when my source tablespace is not ASSM due to the note 1426979. Casually Carsten Leidner made a question in this forum yesterday (you answered him too) and he explained that it's possible to convert fields LONG to LOB with not ASSM tablespaces and in the same step, doing the migration in a new ASSM tablespace.
I'll try to that and when I finish (not very early) I'll write down here the results.
Regards.