Does an employer have an obligation to inform a fellow employee whose status is about to be changed from a non-exempt to an exempt position that his/her accumulated sick leave will be forfeited with the switch?
This company also allows its employees to exchange unused sick leave for cash.

This is governed by state law - most states have a set of laws called "Wage and Hour" laws that govern what an employer can do with wages, and vacation and sick leave. There probably is something that says the employer must disclose the rules relating to sick leave and has to follow them, but you'd have to look at your state's laws.

A 30 day resignation is turned in 3 weeks before a scheduled raise in base pay. Is the employee entitled to the raise during the remaining work days applicable and at what rate would the PTO days be paid at termination?