Router/Firewall Transition Assistance

Hey all,

This may be trivial, as I am very new to this stuff.I am transitioning our organization from using a sonicwall to edgerouter. Before I am entirely ready to go through with the transition I need to know a little more about DHCP, and thought y'all would be a good source of knowledge.

1. How/where/when does the "DHCP clock" (for lack of knowledge of a better word) reset? In the sense that if I were to make the transition how long until all of our devices get their IPs (for those that are dynamic)?
2. If this "clock" takes a set period of time (for right now let's say 24hrs) what problems, if any, arise on all of the devices which have already been assigned IP's in the last 24hrs? Will their IP's transfer over to the new range (the new range is identical to the current) or will they all be confused (and require a renewal of IP's at each device)?

Sorry if this is sadly simple, but I appreciate any assistance y'all can provide.

Thanks in advance,Zealot