Odd. Who did you switch to? I only ask because some platforms like youtube use a form of 'redirect' with their URLs. I would guess yours doesn't if they gave you a m3u link, but possibly with something like a webcasting service they might (Brightcove/Vidyo?)
I would do a quick search of their forums too, for any similar issues. You're saying you only changed the URL from the old one to the new, right? And something IS currently streaming, right? Has it been 24/7? Because Roku only 'reads' your feed every 6 hours, so you can't really pick & choose when to go live unless you time it right.
Lastly, maybe try changing the ID# to something new. Unlikely, but maybe its a cache issue.