Query on secondement of articles

I have joined articles on April 18 2014 and I am prepared to take secondment on April 19 2016, for one year.After that time from whom I can get a certificate regarding completion of articles,and in the meantime can my principal can take an articles in my place since he has the permission to take only 3 articles.Even after completion of secondment do I need to go to my previous principal and re-register under him to complete the articles procedure