The change in lease time should not be a problem. As long as the client device thinks it has a lease, even tho it is longer than the lease time on the dhcp server, it should not lose the connection.
The dhcp server (last I checked) will check to see if an IP is being used before issuing that IP from the dhcp pool. It pings that IP before assigning it to a device. If it gets a ping response on that IP, it marks that IP as "busy" in the lease list, and gets another IP from the pool.
You can check this if you wish.
/ip dhcp-server lease
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