Here are the steps required.
1. Create a second league with the same number of teams as the one you want to change the year with. Create the second league with the desired year.
2. From the Edit FA screen in the setup Delete all the FA (which at the start of the league would be all players). This step may not be required but I would do so to avoid any problems.
3. From the first league, in the set up league menu, misc. Export the roster, ratings and player general files.
4. Open the player general and change the year of birth to be the difference between the first and second league. In my test I had the first league start in 2005 and the second in 2055. I just opened the player general in excel and inserted a column next to the birth date and added the formula d2+18262, with d2 being the old birthdate and the 18262 being the number of days to add to the date (50 years * 365.25 days). I then copied this value to all the rows and then just copied the new birth date and pasted special by value into the records in the d# cells. You must also format the ht to M-D. If you don't have Excel or Open Office then you will need to change each birth date using Notepad and the replace function. (example replace text "/1979" with "/2029". Just find the high and low values in the birthdates and modify the find and replace text values.
5. Copy the following files from the first league into the second- all the csv files that you exported, the player.dat files and the team.dat files. Make sure to import the changed player_general file into the new league. I am not sure if you need to import the other files but I did just "IN CASE" I needed to. I suspect that the player.dat files will have the ratings and that the team files will have the rosters.
This worked for me as long as the same number of teams are in each league.
I hope this helps, remember to BACKUP though!!!!!!!