You can. But how will other servers know that you changed the port? Mail is routed by DNS records. DNS records do not define port number.
Comcast wants you to pay for a commercial line. Thus the limitation on residential services. Plus, under your current service, you need to deal with a dynamic address. Easiest path is to get a business class line.
This is a common question; you're not the first person to have considered resetting the SMTP TCP port.
As Strontium90 indicates, you can most certainly set up your own global mail infrastructure on a variant port (possibly even by using DNS SRV records to find the port de jour), but by the time that's all coded and debugged and generally accepted and standardized and in widespread use around the planet, Comcast and the other ISPs will probably just detect and block the new port or the new service on their residential service tiers. Which means you'll need to add TLS encryption and potentially random ports, and down the proverbial rabbit hole we go.
But seriously, the usual and preferred and best approach is a business class service, as that has port access and you can get correct DNS. Without correct forward and reverse DNS translations, many other SMTP mail servers will detect a dynamic IP service as a spam engine, and drop outbound mail as it arrives. Some mail servers will detect this and will also drop inbound mail, as well.
As an alternative to a business-class service, you can set up a mail relay through your ISP, or potentially a relay via a commercial mail relay service — if either of these approaches is permitted by your ISP terms of service. There are various folks that offer these mail hop services by subscription, but then all your mail is routed through that service and that may or may not be desirable.
Hosting your own virtual private server or dedicated host just for mail can be pretty cheap these days, if you're willing to run your mail server in a data center. That avoids all entanglements with your ISP. (But again, if you're willing to host your mail elsewhere.)
I don't have a dynamic fyi. Comcast has told us that they keep the same IP for us unless we request otherwise. Our IP has never changed. I do know comcast tells us to use smtp.comcast.net to relay, but I tried that (I THINK I did it correctly, not 100% sure), and it still failed.
Also the whole point we want a mail server is so we can have the mail stored locally.