I have a doubt. Can access point communicating with a client on a low frequency channel(say 2.4GHz) change to high frequency channel(say 60GHz) once the client is within range? Is it possible from AP side (or) client side (or) both (or) neither?
Enterprise wireless equipment (Cisco/Aruba/Xirrus/Ruckus etc) already does this. Different vendors have different names for it but 'active band steering' is a commonly used term.
Most client devices will connect to the strongest available signal at 2.4GHz and stay there. Within certain parameters the wireless system will then try and force the client to associate using a different frequency (in WiFi typically 5GHz channels) by sending disassociation packets or refusing connection attempts on the 2.4GHz radio. Some vendors allow you to configure the 'aggression' level which is used (how many times the system refuses 2.4GHz connection attempts).
Client devices are dumb, the network is smart. There will be some outlier devices (old smartphones / pos scanners etc) that just won't connect with band steering enabled.
This post from meraki explains in more detail.