Is it possible to upgrade to an F.O on the frieghter fleet after being an S.O for a while ??
Yes
If yes how long does it normally take ??
Pretty much as soon as a notice to crew comes out advertising vacancies for freighter FO's in certain bases.
Is this something out of the ordinary / has it been done before ??
Probably a little out of the ordinary for a few reasons. Firstly, I think that alot of SO's don't realise that they can bid for a Freighter FO spot on a base. Secondly, they may be happy with the SO roster or living in HKG with the various benefits and don't want to go. Yes, it has been done before.
There are plenty of factors to consider, but plenty of options also. Certainly, if you join the SO route, but want to get back to the US quicker than you would via the SO-JFO-FO route....bidding as described above is an option.