Hiding UAN of previous employer to new employer - What exactly will happen if I say I forgot my UAN?

What exactly will happen if I say I forgot my UAN or i don't have UAN to the new employer.... i come to know the system wont allow to generate new Uan but what happen next... or if i provide my uan isnt the employer can able to find my previous employment details...do they consider that for bgv

aadhar linked uan's are auto displayed in system and system prevent the employer to generate new but ask them to use the old one