Can you provide further details as to what sort of cable would need to be purchased to make changes to the message? Also is there any software to do so?
RS232 Serial Port (COM PORT) is the computer side, and the sign side looks like a standard 6-pin RJ-11 (?). Whatever the 6-pin equivalent of RJ-45 is. I've seen lots of these types of cables used on UPS battery backups, and I would imagine you could just use a cable from one of those. I do not have any software with it though.
Also, did some searching and found a PDF of the spec sheet and the manual for it.http://www.webstaurantstore.com/PDF/TruColorIIManual.pdfhttp://www.webstaurantstore.com/specsheets/PROLITETRUCOLORII.PDF
I'll call up Pro-Lite when I'm not at work and see what it would take to get a remote and/or software for it. It sounds like the remote is the most common way to program the sign.