Can I obtain my COS?

Hi all,

Recently graduated from uni and my student visa (tier 4) expires on May 15 2018.

Last week I received a graduate job offer (~27k) in London and the company is willing to sponsor me, but I don't think they can get a COS in time.

I have my flight booked back to my country and I'm planning on applying from there, will I be qualified to "switch" from tier 4 to tier 2, or will I have to apply as a new entrant for the RCOS which I hear is highly competitive lately and I'm nervous it will be rejected.

The job starts in September so I still have time but I'm quite worried!

Thanks