The UCI assigned to you will be the same whether it’s for a temporary or permanent visa application. So mention your old UCI.I lived in Canada for one year with a working holiday visa (International Experience Canada).
Now I'm filling my express entry profile, and it is asking me if I applied to IRCC before, I put yes to that question.
And after that, it asks me if I have a UCI, and I had a UCI for my IEC but I'm not really sure that that UCI is still valid, and it is not an Express Entry UCI.
Should I leave that field blank?