Transfering Phone Number & Whattsapp Access

Hello

This is a strange scenario.

Without sharing too much detail the police have my phone as part of an investigation (dont worry im innocent - they just needed to see some whattsapp messages from another person)

I have a new phone and I want to transfer my number as its my business number and my van and cards etc all have that number on and it will be expensive to change it all.

If I transfer the number to my old phone, will the police still be able to access the whattsapp messages? I know texts would be saved on the phone but with whattsapp being online I dont want to hinder their access.

Any help?

Thanks