Help with proving not market renter

Just wondering where we stand. I'm housing association 3 bedroom found exchange to 2 bedroom. Need to downsize as only 3 adults  now so have spare room. The lady I'm wanting to swap with is York council 2 bed. York are OK with swap as she has 3 kids and one on way so my house perfect. Yorkshire housing refusing as saying we are market renters. We were on a 5 year affordable assured shorthold tenancy till it ran out then on periodic rolling monthly. So not market renter. Both have excellent credit scores, no rent arrears not asking for bigger or smaller than need. Not been adapted etc. Any help very much appreciated 👍