The first phase of the transition is to replace the single point of failure: your legacy master. It's hardly a solid performer in today's monitoring environment, and odds are that you don't have a replacement available. If your master fails, you'll lose all visibility, and the value of your legacy RTUs will effectively reduce to nothing.
"Our DEC PDP-11 master computer crashed on Monday. We contacted the original alarm system manufacturer, only to learn that our master was discontinued and no longer supported. In desperation, we called DPS. A technician arrived with a new master the following Tuesday and we were back in operation the next day."
-Phil Shew, PNM Electric Services
Here's the secret: a new multi-protocol master. If you're like most people, you're trying to figure out how you can justify an all-at-once "wholesale swapout" of both your master station AND all of your RTUs. But imagine a master station that can work with your legacy RTUs AND modern protocols like SNMP, DNP3, etc. That would be an empowering tool. You could ditch your old master (it probably won't last much longer anyway), keep your working RTUs, and deploy new RTUs at your own pace (whenever you have a failure or you get some budget money).
"T/Mon has been excellent to work with, making interfacing with our current Pulsecom polling units and our 10D dial-up units in the field 100% reliable... Everything was initially setup and working in one day."
-David Wagner, Niagara Mohawk
Even if you have a modern master that you'd like to keep as your top-level monitoring device, a new multi-protocol master will let you do that. You can be deploy the new master as an element manager, mediating its alarms (as SNMP traps or another protocol) to your top-level master (Manager of Managers, "MOM").
"Our existing fleet of Datalok 10A's appear basically unaffected by the change to a new master polling device... If anything, they may be a little happier."
-Bryce Pursley, Duke Energy