I have changed a Galaxy to 7 digit and it is not too hard, but;

• You need to be able to burn the ROM's or know someone who can.
  You need access a crimper to make up the extra wire you need to add to the loom.
  You need 4 Bally or Stern 7 digit displays which can get expensive.

Another option is to change the settings on the dip-switches to make the game harder - 3 balls and 'conservative' setting (see the manual).

cheers
Ian