this is a MUST procedure learned from experience.
when you get there, upload the program and save it.
copy it to another location as a backup.
the program i the plc is the master, you may not have the latest working program, timer, counter, data values.
COMPARE what you started with (hopefully you still have a copy) to see the differences, program, timers, counters, data.
modify your program accordingly.
hope this helps,
james