The criteria:
SDR/MDR Profiles can change the Lead Source field from "Outbound" to "Inbound" only when the Funnel Stage is "MQL" and the Funnel Status is "Open" or "Engaged". They cannot change the Lead Source if the starting value is anything other than Outbound.
What I have right now that is not working:
AND(OR (ISCHANGED(LeadSource),
PRIORVALUE(LeadSource) <> "Outbound",
NOT (ISPICKVAL(LeadSource , "Inbound")),
NOT (ISPICKVAL(Funnel_Stage__c , "MQL")),
($Profile.Name <> "SDR"),
NOT(OR (ISPICKVAL ( Funnel_Status__c , "Open"))),
(ISPICKVAL ( Funnel_Status__c , "Engaged")))))
Hi @Mike Miller and @Cătălin Duță - Thank you both so very very much, with absolutely sincerity. I just tried the last suggestion by Catalin and so far everything I've tried has been successful!! Tyler is out sick today but I'll review the tests with him when he returns for him to run through his test scenarios as well. From a UAT perspective this passes but will wait for both Dev and QA to sign off as I'm jumping ahead.
You both have been a tremendous help. We can not thank you enough. If I don't post again on this topic, I hope you both have a fantastic new year!