IB Credits ESA or IS

I have a client who is in receipt of IBcredits longterm and was in receipt of IS and then was advised to claim JSA when her partner moved in.
They then separated, Jobcentre assisted with a new claim IS but refused and advised to claim ESA.

DWP are now saying the rules have changed but I cannot find this info out, does she have to claim ESA or should she be able to return to IS?

Under the Employment and Support Allowance (Transitional Provisions) Regulations 2008, some of which has since been revoked, she cannot reclaim IS unless she either gets ICB or SDA.  Or, under the IS Gen’ Regs, she falls within another prescribed category of person who can claim IS.  For instance, single parents, carers, etc. 

I’m not aware of any further revocation but someone might put me right on that.  However, even if she were entitled, is there anything stopping them making a positive IS decision then immediately converting the award to an award of ESA?