I want to change estate agent, is the HIP commissioned by my first agent valid for use by the new agent?

Yes. A HIP is valid as long as it legally compliant and there has been no break in marketing. However, the question of whether you can transport the HIP with you depends on the ownership of the HIP and the commercial terms that  were agreed with the estate agent or HIP provider. A Fridaysmove HIP belongs to the seller and only the seller.