Sure,
you can purge the cylinders by loosening/removing one end of the lines on the "return" from the cylinders. Put it in a bucket to catch the oil and then start the tractor up and move the cylinders to push it out. This calls for some planning so you don't get an oil bath! Lots of flow there if you move the cylinder full speed, so tie the end of the return line into the bucket, tie the bucket etc. Cylinders with valves in the pistons will allow oil to flow through the piston when the piston comes to the end of the stroke, so stop pushing when you begin to get clean oil out the hoses.
note, this is not the return line from the valve if you have a remote valve, separate from the pump, but the lines from one end of the cylinders to the valve.
Second note, If you use the tractor pump and push the cylinders with pressure, do both cylinders at the same time and when you have done one end, then you must hold the cylinder position, tighten the first lines and remove the ones on the other end and put in the bucket. May need to refill the tank with clean oil before you do the other end.
Probably easier to remove the pin from the rod end of the cylinder and tie/block it up so it can go full stroke without hitting anything before you start. That way you don't have to have a jack to hold the blade and/or tractor up while you replace the lines.
If you have some way to stroke the cylinder without cranking the tractor up and doing it that way, it would be safer, but less complete. I would not use air pressure to move the pistons. High pressure air may result in sudden movement of the cylinder and may damage things if the pistons slam into the heads, or hurt someone if they get in the way of the rod as it jumps out.
Like I said, the quick and dirty way!
D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time:D