I had a good friend get sick of the armd life and went to a blue trade.After a year and a bit there he realised he was better suited back where he came from,Summitted a memo,after a while for the paper work to go through and a whole lot of attention in front of desk he was back.However your old trade doesn't have to take you back,the BPSO should have told you that when you left it.
Also it's a good point Mr O'Leary put up.Also,If it was 10 years ago,you may not meet the requirements for your old trade anymore.