Find specific text and substitute characters

I've attached a sample spreadsheet. In Column C the alpha prefixes will be followed by policy numbers. The alpha prefixes can be several different policy types (LX, OX, LCLP, GUAR, etc.) I want to find only those cells that begin with either "LX" or "OX" and substitute "LXOR" for LX and "OXOR" for OX followed by the policy number. NOTE: Column D has been added to show what the prefix/policy number will look like after substitution.

For example: LX123456789 will now become LXOR123456789

The number of rows in Column C will vary from spreadsheet to spreadsheet.