Chiming in with my experience. 3 homes with 50mb Time Warner Cable (TWC) and became Spectrum. 100mb is the minimum advertised speed in my area and costs just $5 more.
On my 1st call for my home I asked for the 100mb price for $5 more and was told that it was not available for me since I was an existing customer. They were kind enough to let me work things out with retentions. Ended up getting 200mb for about $10 more than 100mb or $15 more to go from 50 to 200 for 1 year only. I took it. Found 200mb useless since wireless cards in my laptops can only achieve it if I am in the same room with line of sight to the router. After 3 months, I called to reduce my speed to 100mb/s. That's when they finally was able to give me the 100mb/s price for $5 more than my 50mb/s price with TWC. That's also when they provided a new cable/WiFi router at their expense. TWC used to charge $10 a month for the combo/WiFi. I don't use the built in WiFi anyhow and set the cable modem to bridge mode.
2nd home wasn't bad. They had Internet/phone and wanted to add cable. They had no issues giving the free upgrade to 100mb/s with no price increase probably since they are making more money per month with a new cable subscription.
3rd home was probably 4 months after the 2nd home. They were charging that home the $10 cable modem/WiFi fee when never had such a box. We used our own cable modem. They were more than happy to reduce the monthly charge to make up for the $10 charge and upgrade them to 100mb/s.
Lessons learned... It wasn't straightforward to get the free upgrade. If they say no, temporarily upgrade your internet to beyond 100mb/s. Use it for a month or two then call again to drop your speed. You'll have a better chance with retentions.
Most important lesson. Realistically with real-world wireless signals and mobile device speeds 75mb/s is the sweet spot. @50mb/s service, my speedtests were 69mb/s, but it's hard to deny 100mb/s when it is just $5 more and my average speedtest is 120.