Taken that most times the log file rotation is set on the by midnight timeframe, there would be at least 1 file per day. So when you copy those files over to the new R80 management server you would to set the number of days to the number calculated by:
(todays date) - (first date of imported files) so when you imported files starting at Feb 1st you should use the number 25
It will works it way back from the Feb 25th to Feb 1st, there will be a number of days (since your upgrade) that already have been indexed and they will be skipped.
Do keep in mind this is a very time consuming job and will not finish the same day when you have many files.
Regards, Maarten