Unless you already had something implemented for reading and logging the IP, you won't be finding a log of previous IPs.
Best way for you to be certain right now is to visit IP Burger - What is my IP Address? Find Your IP Address! then change the router mac address, then unplug the cable modem for 5-20 seconds, plug it back in and visit IP Burger - What is my IP Address? Find Your IP Address! again to SEE that the IP did change.
Don't be too quick to take his word for the service change. He's biased by his desire to get your money in his wallet continually. Might say anything, just to keep you thinking you are content.
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