The extent of the task's difficulty depends greatly on the type of programming employed. Obvioulsy the program logic must be reentered to go to any other platform. You can do anything with the AutomationDirect.com (ADC) PLC you can with Allen Bradley (A-B), and each has its strengths and weaknesses.
If the A-B program is primarily straight ladder logic with sequencing, discrete logic, and timers & counters then the conversion won't be difficult. If the program uses a lot of "tricks" like bit shifting and compute blocks and indirect adressing the task is much more difficult.
If you have an operator interface or communications network the impact on these peripherals must also be considered.
The ADC is easier for setting up adressing and documentation, and the communications is simpler. The A-B has better math functions and "advanced" program functions. My experience is that developing a program from scratch and debugging is easier in the ADC because the programming interface is simpler.