The approach to get the previous record's value in Telerik Reporting is via SQL code in the query to the database (or programmatically inside your business object if ObjectDataSource
For example, you can calculate the difference between the current row value and the previous row value and this is how such SQL query will look when using the AdventureWorks
(t1.SickLeaveHours - t2.SickLeaveHours, 0)
[Difference], t1.EmployeeID, t1.Title, t1.SickLeaveHours
t1.EmployeeID = t2.EmployeeID + 1
Then use the Difference
data field in conditional formatting rules to change the style of the row.